What is the coefficient of $x^{11}$ in $(3x-9)^{19}$?












1












$begingroup$


I am currently studying for finals, and I do not know how to do this problem from my study guide. I have tried to watch a few YouTube videos and I know that I will end up with $3x^{11} times (-9)^8$, but from here I do not know where to go. The YouTube videos I have watched have been unclear, and I do not have a chance to ask my professor until just a little before the exam. If anyone has any tips on where to go from here I would greatly appreciate it!










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$endgroup$












  • $begingroup$
    Don't forget about the binomial coefficient.
    $endgroup$
    – John Wayland Bales
    Dec 7 '18 at 16:40










  • $begingroup$
    Are you aware of the BInomial expansion ${(ax-b)}^n = sum ^ n _ {r=0} binom{n}{r} {(ax)}^{n-r} cdot (-1)^r cdot b^r$?
    $endgroup$
    – Prakhar Nagpal
    Dec 7 '18 at 16:41


















1












$begingroup$


I am currently studying for finals, and I do not know how to do this problem from my study guide. I have tried to watch a few YouTube videos and I know that I will end up with $3x^{11} times (-9)^8$, but from here I do not know where to go. The YouTube videos I have watched have been unclear, and I do not have a chance to ask my professor until just a little before the exam. If anyone has any tips on where to go from here I would greatly appreciate it!










share|cite|improve this question











$endgroup$












  • $begingroup$
    Don't forget about the binomial coefficient.
    $endgroup$
    – John Wayland Bales
    Dec 7 '18 at 16:40










  • $begingroup$
    Are you aware of the BInomial expansion ${(ax-b)}^n = sum ^ n _ {r=0} binom{n}{r} {(ax)}^{n-r} cdot (-1)^r cdot b^r$?
    $endgroup$
    – Prakhar Nagpal
    Dec 7 '18 at 16:41
















1












1








1





$begingroup$


I am currently studying for finals, and I do not know how to do this problem from my study guide. I have tried to watch a few YouTube videos and I know that I will end up with $3x^{11} times (-9)^8$, but from here I do not know where to go. The YouTube videos I have watched have been unclear, and I do not have a chance to ask my professor until just a little before the exam. If anyone has any tips on where to go from here I would greatly appreciate it!










share|cite|improve this question











$endgroup$




I am currently studying for finals, and I do not know how to do this problem from my study guide. I have tried to watch a few YouTube videos and I know that I will end up with $3x^{11} times (-9)^8$, but from here I do not know where to go. The YouTube videos I have watched have been unclear, and I do not have a chance to ask my professor until just a little before the exam. If anyone has any tips on where to go from here I would greatly appreciate it!







exponential-function binomial-coefficients binomial-theorem






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edited Dec 7 '18 at 17:10









PM.

3,3532925




3,3532925










asked Dec 7 '18 at 16:36









bmurf17bmurf17

104




104












  • $begingroup$
    Don't forget about the binomial coefficient.
    $endgroup$
    – John Wayland Bales
    Dec 7 '18 at 16:40










  • $begingroup$
    Are you aware of the BInomial expansion ${(ax-b)}^n = sum ^ n _ {r=0} binom{n}{r} {(ax)}^{n-r} cdot (-1)^r cdot b^r$?
    $endgroup$
    – Prakhar Nagpal
    Dec 7 '18 at 16:41




















  • $begingroup$
    Don't forget about the binomial coefficient.
    $endgroup$
    – John Wayland Bales
    Dec 7 '18 at 16:40










  • $begingroup$
    Are you aware of the BInomial expansion ${(ax-b)}^n = sum ^ n _ {r=0} binom{n}{r} {(ax)}^{n-r} cdot (-1)^r cdot b^r$?
    $endgroup$
    – Prakhar Nagpal
    Dec 7 '18 at 16:41


















$begingroup$
Don't forget about the binomial coefficient.
$endgroup$
– John Wayland Bales
Dec 7 '18 at 16:40




$begingroup$
Don't forget about the binomial coefficient.
$endgroup$
– John Wayland Bales
Dec 7 '18 at 16:40












$begingroup$
Are you aware of the BInomial expansion ${(ax-b)}^n = sum ^ n _ {r=0} binom{n}{r} {(ax)}^{n-r} cdot (-1)^r cdot b^r$?
$endgroup$
– Prakhar Nagpal
Dec 7 '18 at 16:41






$begingroup$
Are you aware of the BInomial expansion ${(ax-b)}^n = sum ^ n _ {r=0} binom{n}{r} {(ax)}^{n-r} cdot (-1)^r cdot b^r$?
$endgroup$
– Prakhar Nagpal
Dec 7 '18 at 16:41












4 Answers
4






active

oldest

votes


















1












$begingroup$

$$(3x-9)^{19} = (3x-9)(3x-9) cdots (3x-9) $$



Notice that on the right hand side there exist $19$ products. A term can be formed by choosing either $3x$ or $-9$ from each of these $19$ products.



If your goal is to find the coefficient of $x^{11}$, you have to choose $3x$ for a $11$ times from the available $19$ products, and then choose $-9$ for $8$ times from the remaining $8$ products.



If you choose $3x$ for $11$ times, you end up with $color{blue}{(3x)^{11}}$, and you can do this in exactly $color{green}{binom{19}{11}}$ ways. $color{grey}{text{(because you have $19$ products available and you're considering only $11$ of them for $3x$.)}}$



For each of the above $color{green}{binom{19}{11}}$ ways, you can choose $-9$ from the remaining $8$ products, this gives $color{red}{(-9)^8}$

So the overall term would be $color{green}{binom{19}{11}}color{blue}{(3x)^{11}}color{red}{(-9)^8}$






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Thank you this makes sense! I completely forgot about the choose
    $endgroup$
    – bmurf17
    Dec 7 '18 at 19:04



















0












$begingroup$

HINT:
The Binomial expansion : $$(a+b)^{n}= sum_{k=0}^{n} {n choose k}{a}^{k}{b}^{n-k}.$$



Substitute $a=3x$ and $b=-9$ . To find out the coefficient of $x^{11} $,put $k=11$ in ${n choose k}{a}^{k}{b}^{n-k}$






share|cite|improve this answer









$endgroup$





















    0












    $begingroup$

    Note that by expansion of a binomial to the $n^{th}$ power, you get



    $$(a+b)^n = sum_{r = 0}^{n} {n choose r}a^{n-r}b^r = a^n+a^{n-1}b+a^{n-2}b^2+…+a^{n-r}b^r+…+ab^{n-1}+b^n$$



    where ${n choose r}$ corresponds to the binomial coefficients, or Pascal Numbers. (Never forget those for coefficients.)



    For $(3x-9)^{19}$, the first term has $x^{19}$, the second term has $x^{18}$, etc. So, $x^{11}$ corresponds to the ninth term. Hence, you get



    $${19 choose 19-8} (3x)^{11}(-9)^8$$



    $${19 choose 11} (3x)^{11}9^8$$



    Simplifying from here, you can solve for the ninth term and find the coefficient of $x^{11}$.






    share|cite|improve this answer











    $endgroup$





















      0












      $begingroup$

      You are part way there.



      If you were to write out $(3x-9)^{19}$ in full, it would be a product of nineteen $ (3x-9)$. To get an $x^{11}$ you would need to pick 11 of these $(3x-9)$.



      There are $19 choose 11$ ways of doing that, so the term in $x^{11}$ is
      $$
      {19 choose 11} (3x)^{11} (-9)^8
      $$



      Thus the coefficient of $x^{11}$ is



      $$
      {19 choose 11} (3)^{11} (-9)^8 = {19 choose 11}3^{27}
      $$






      share|cite|improve this answer









      $endgroup$













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        4 Answers
        4






        active

        oldest

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        4 Answers
        4






        active

        oldest

        votes









        active

        oldest

        votes






        active

        oldest

        votes









        1












        $begingroup$

        $$(3x-9)^{19} = (3x-9)(3x-9) cdots (3x-9) $$



        Notice that on the right hand side there exist $19$ products. A term can be formed by choosing either $3x$ or $-9$ from each of these $19$ products.



        If your goal is to find the coefficient of $x^{11}$, you have to choose $3x$ for a $11$ times from the available $19$ products, and then choose $-9$ for $8$ times from the remaining $8$ products.



        If you choose $3x$ for $11$ times, you end up with $color{blue}{(3x)^{11}}$, and you can do this in exactly $color{green}{binom{19}{11}}$ ways. $color{grey}{text{(because you have $19$ products available and you're considering only $11$ of them for $3x$.)}}$



        For each of the above $color{green}{binom{19}{11}}$ ways, you can choose $-9$ from the remaining $8$ products, this gives $color{red}{(-9)^8}$

        So the overall term would be $color{green}{binom{19}{11}}color{blue}{(3x)^{11}}color{red}{(-9)^8}$






        share|cite|improve this answer











        $endgroup$













        • $begingroup$
          Thank you this makes sense! I completely forgot about the choose
          $endgroup$
          – bmurf17
          Dec 7 '18 at 19:04
















        1












        $begingroup$

        $$(3x-9)^{19} = (3x-9)(3x-9) cdots (3x-9) $$



        Notice that on the right hand side there exist $19$ products. A term can be formed by choosing either $3x$ or $-9$ from each of these $19$ products.



        If your goal is to find the coefficient of $x^{11}$, you have to choose $3x$ for a $11$ times from the available $19$ products, and then choose $-9$ for $8$ times from the remaining $8$ products.



        If you choose $3x$ for $11$ times, you end up with $color{blue}{(3x)^{11}}$, and you can do this in exactly $color{green}{binom{19}{11}}$ ways. $color{grey}{text{(because you have $19$ products available and you're considering only $11$ of them for $3x$.)}}$



        For each of the above $color{green}{binom{19}{11}}$ ways, you can choose $-9$ from the remaining $8$ products, this gives $color{red}{(-9)^8}$

        So the overall term would be $color{green}{binom{19}{11}}color{blue}{(3x)^{11}}color{red}{(-9)^8}$






        share|cite|improve this answer











        $endgroup$













        • $begingroup$
          Thank you this makes sense! I completely forgot about the choose
          $endgroup$
          – bmurf17
          Dec 7 '18 at 19:04














        1












        1








        1





        $begingroup$

        $$(3x-9)^{19} = (3x-9)(3x-9) cdots (3x-9) $$



        Notice that on the right hand side there exist $19$ products. A term can be formed by choosing either $3x$ or $-9$ from each of these $19$ products.



        If your goal is to find the coefficient of $x^{11}$, you have to choose $3x$ for a $11$ times from the available $19$ products, and then choose $-9$ for $8$ times from the remaining $8$ products.



        If you choose $3x$ for $11$ times, you end up with $color{blue}{(3x)^{11}}$, and you can do this in exactly $color{green}{binom{19}{11}}$ ways. $color{grey}{text{(because you have $19$ products available and you're considering only $11$ of them for $3x$.)}}$



        For each of the above $color{green}{binom{19}{11}}$ ways, you can choose $-9$ from the remaining $8$ products, this gives $color{red}{(-9)^8}$

        So the overall term would be $color{green}{binom{19}{11}}color{blue}{(3x)^{11}}color{red}{(-9)^8}$






        share|cite|improve this answer











        $endgroup$



        $$(3x-9)^{19} = (3x-9)(3x-9) cdots (3x-9) $$



        Notice that on the right hand side there exist $19$ products. A term can be formed by choosing either $3x$ or $-9$ from each of these $19$ products.



        If your goal is to find the coefficient of $x^{11}$, you have to choose $3x$ for a $11$ times from the available $19$ products, and then choose $-9$ for $8$ times from the remaining $8$ products.



        If you choose $3x$ for $11$ times, you end up with $color{blue}{(3x)^{11}}$, and you can do this in exactly $color{green}{binom{19}{11}}$ ways. $color{grey}{text{(because you have $19$ products available and you're considering only $11$ of them for $3x$.)}}$



        For each of the above $color{green}{binom{19}{11}}$ ways, you can choose $-9$ from the remaining $8$ products, this gives $color{red}{(-9)^8}$

        So the overall term would be $color{green}{binom{19}{11}}color{blue}{(3x)^{11}}color{red}{(-9)^8}$







        share|cite|improve this answer














        share|cite|improve this answer



        share|cite|improve this answer








        edited Dec 7 '18 at 17:12

























        answered Dec 7 '18 at 17:00









        rsadhvikarsadhvika

        1,7101228




        1,7101228












        • $begingroup$
          Thank you this makes sense! I completely forgot about the choose
          $endgroup$
          – bmurf17
          Dec 7 '18 at 19:04


















        • $begingroup$
          Thank you this makes sense! I completely forgot about the choose
          $endgroup$
          – bmurf17
          Dec 7 '18 at 19:04
















        $begingroup$
        Thank you this makes sense! I completely forgot about the choose
        $endgroup$
        – bmurf17
        Dec 7 '18 at 19:04




        $begingroup$
        Thank you this makes sense! I completely forgot about the choose
        $endgroup$
        – bmurf17
        Dec 7 '18 at 19:04











        0












        $begingroup$

        HINT:
        The Binomial expansion : $$(a+b)^{n}= sum_{k=0}^{n} {n choose k}{a}^{k}{b}^{n-k}.$$



        Substitute $a=3x$ and $b=-9$ . To find out the coefficient of $x^{11} $,put $k=11$ in ${n choose k}{a}^{k}{b}^{n-k}$






        share|cite|improve this answer









        $endgroup$


















          0












          $begingroup$

          HINT:
          The Binomial expansion : $$(a+b)^{n}= sum_{k=0}^{n} {n choose k}{a}^{k}{b}^{n-k}.$$



          Substitute $a=3x$ and $b=-9$ . To find out the coefficient of $x^{11} $,put $k=11$ in ${n choose k}{a}^{k}{b}^{n-k}$






          share|cite|improve this answer









          $endgroup$
















            0












            0








            0





            $begingroup$

            HINT:
            The Binomial expansion : $$(a+b)^{n}= sum_{k=0}^{n} {n choose k}{a}^{k}{b}^{n-k}.$$



            Substitute $a=3x$ and $b=-9$ . To find out the coefficient of $x^{11} $,put $k=11$ in ${n choose k}{a}^{k}{b}^{n-k}$






            share|cite|improve this answer









            $endgroup$



            HINT:
            The Binomial expansion : $$(a+b)^{n}= sum_{k=0}^{n} {n choose k}{a}^{k}{b}^{n-k}.$$



            Substitute $a=3x$ and $b=-9$ . To find out the coefficient of $x^{11} $,put $k=11$ in ${n choose k}{a}^{k}{b}^{n-k}$







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered Dec 7 '18 at 16:48









            Thomas ShelbyThomas Shelby

            3,9542625




            3,9542625























                0












                $begingroup$

                Note that by expansion of a binomial to the $n^{th}$ power, you get



                $$(a+b)^n = sum_{r = 0}^{n} {n choose r}a^{n-r}b^r = a^n+a^{n-1}b+a^{n-2}b^2+…+a^{n-r}b^r+…+ab^{n-1}+b^n$$



                where ${n choose r}$ corresponds to the binomial coefficients, or Pascal Numbers. (Never forget those for coefficients.)



                For $(3x-9)^{19}$, the first term has $x^{19}$, the second term has $x^{18}$, etc. So, $x^{11}$ corresponds to the ninth term. Hence, you get



                $${19 choose 19-8} (3x)^{11}(-9)^8$$



                $${19 choose 11} (3x)^{11}9^8$$



                Simplifying from here, you can solve for the ninth term and find the coefficient of $x^{11}$.






                share|cite|improve this answer











                $endgroup$


















                  0












                  $begingroup$

                  Note that by expansion of a binomial to the $n^{th}$ power, you get



                  $$(a+b)^n = sum_{r = 0}^{n} {n choose r}a^{n-r}b^r = a^n+a^{n-1}b+a^{n-2}b^2+…+a^{n-r}b^r+…+ab^{n-1}+b^n$$



                  where ${n choose r}$ corresponds to the binomial coefficients, or Pascal Numbers. (Never forget those for coefficients.)



                  For $(3x-9)^{19}$, the first term has $x^{19}$, the second term has $x^{18}$, etc. So, $x^{11}$ corresponds to the ninth term. Hence, you get



                  $${19 choose 19-8} (3x)^{11}(-9)^8$$



                  $${19 choose 11} (3x)^{11}9^8$$



                  Simplifying from here, you can solve for the ninth term and find the coefficient of $x^{11}$.






                  share|cite|improve this answer











                  $endgroup$
















                    0












                    0








                    0





                    $begingroup$

                    Note that by expansion of a binomial to the $n^{th}$ power, you get



                    $$(a+b)^n = sum_{r = 0}^{n} {n choose r}a^{n-r}b^r = a^n+a^{n-1}b+a^{n-2}b^2+…+a^{n-r}b^r+…+ab^{n-1}+b^n$$



                    where ${n choose r}$ corresponds to the binomial coefficients, or Pascal Numbers. (Never forget those for coefficients.)



                    For $(3x-9)^{19}$, the first term has $x^{19}$, the second term has $x^{18}$, etc. So, $x^{11}$ corresponds to the ninth term. Hence, you get



                    $${19 choose 19-8} (3x)^{11}(-9)^8$$



                    $${19 choose 11} (3x)^{11}9^8$$



                    Simplifying from here, you can solve for the ninth term and find the coefficient of $x^{11}$.






                    share|cite|improve this answer











                    $endgroup$



                    Note that by expansion of a binomial to the $n^{th}$ power, you get



                    $$(a+b)^n = sum_{r = 0}^{n} {n choose r}a^{n-r}b^r = a^n+a^{n-1}b+a^{n-2}b^2+…+a^{n-r}b^r+…+ab^{n-1}+b^n$$



                    where ${n choose r}$ corresponds to the binomial coefficients, or Pascal Numbers. (Never forget those for coefficients.)



                    For $(3x-9)^{19}$, the first term has $x^{19}$, the second term has $x^{18}$, etc. So, $x^{11}$ corresponds to the ninth term. Hence, you get



                    $${19 choose 19-8} (3x)^{11}(-9)^8$$



                    $${19 choose 11} (3x)^{11}9^8$$



                    Simplifying from here, you can solve for the ninth term and find the coefficient of $x^{11}$.







                    share|cite|improve this answer














                    share|cite|improve this answer



                    share|cite|improve this answer








                    edited Dec 7 '18 at 17:02

























                    answered Dec 7 '18 at 16:44









                    KM101KM101

                    6,0901525




                    6,0901525























                        0












                        $begingroup$

                        You are part way there.



                        If you were to write out $(3x-9)^{19}$ in full, it would be a product of nineteen $ (3x-9)$. To get an $x^{11}$ you would need to pick 11 of these $(3x-9)$.



                        There are $19 choose 11$ ways of doing that, so the term in $x^{11}$ is
                        $$
                        {19 choose 11} (3x)^{11} (-9)^8
                        $$



                        Thus the coefficient of $x^{11}$ is



                        $$
                        {19 choose 11} (3)^{11} (-9)^8 = {19 choose 11}3^{27}
                        $$






                        share|cite|improve this answer









                        $endgroup$


















                          0












                          $begingroup$

                          You are part way there.



                          If you were to write out $(3x-9)^{19}$ in full, it would be a product of nineteen $ (3x-9)$. To get an $x^{11}$ you would need to pick 11 of these $(3x-9)$.



                          There are $19 choose 11$ ways of doing that, so the term in $x^{11}$ is
                          $$
                          {19 choose 11} (3x)^{11} (-9)^8
                          $$



                          Thus the coefficient of $x^{11}$ is



                          $$
                          {19 choose 11} (3)^{11} (-9)^8 = {19 choose 11}3^{27}
                          $$






                          share|cite|improve this answer









                          $endgroup$
















                            0












                            0








                            0





                            $begingroup$

                            You are part way there.



                            If you were to write out $(3x-9)^{19}$ in full, it would be a product of nineteen $ (3x-9)$. To get an $x^{11}$ you would need to pick 11 of these $(3x-9)$.



                            There are $19 choose 11$ ways of doing that, so the term in $x^{11}$ is
                            $$
                            {19 choose 11} (3x)^{11} (-9)^8
                            $$



                            Thus the coefficient of $x^{11}$ is



                            $$
                            {19 choose 11} (3)^{11} (-9)^8 = {19 choose 11}3^{27}
                            $$






                            share|cite|improve this answer









                            $endgroup$



                            You are part way there.



                            If you were to write out $(3x-9)^{19}$ in full, it would be a product of nineteen $ (3x-9)$. To get an $x^{11}$ you would need to pick 11 of these $(3x-9)$.



                            There are $19 choose 11$ ways of doing that, so the term in $x^{11}$ is
                            $$
                            {19 choose 11} (3x)^{11} (-9)^8
                            $$



                            Thus the coefficient of $x^{11}$ is



                            $$
                            {19 choose 11} (3)^{11} (-9)^8 = {19 choose 11}3^{27}
                            $$







                            share|cite|improve this answer












                            share|cite|improve this answer



                            share|cite|improve this answer










                            answered Dec 7 '18 at 17:04









                            PM.PM.

                            3,3532925




                            3,3532925






























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