$cos^{-1}x+cos^{-1}y=cos^{-1}Big(xy-sqrt{1-x^2}sqrt{1-y^2}Big)$ true for all $x$?
$begingroup$
$cos^{-1}x+cos^{-1}y=cos^{-1}Big(xy-sqrt{1-x^2}sqrt{1-y^2}Big)$ true for all $x$ ?
My Attempt
Let $a=cos^{-1}x$, $b=cos^{-1}yimplies$$cos a=x$, $cos b=y$ and $a,bin[0,pi]implies a+bin[0,2pi]$
$$
cos(a+b)=cos acos b-sin asin b=xy-sqrt{1-x^2}sqrt{1-y^2}\=cosbigg[cos^{-1}Big(xy-sqrt{1-x^2}sqrt{1-y^2}Big)bigg]\
a+b=color{red}{cos^{-1}x+cos^{-1}y=2npipmcos^{-1}Big(xy-sqrt{1-x^2}sqrt{1-y^2}Big)}
$$
Case 1: $a+bin[0,pi)$
$$
cos^{-1}x+cos^{-1}y=cos^{-1}Big(xy-sqrt{1-x^2}sqrt{1-y^2}Big)
$$
Case 2: $a+bin[pi,2pi]$
$$
cos^{-1}x+cos^{-1}y=2pi-cos^{-1}Big(xy-sqrt{1-x^2}sqrt{1-y^2}Big)
$$
Case 1-: $a+bin[0,pi)$
Note I have checked a possible solution to this in link, but it gives a different expression differ from my attempt.
trigonometry inverse-function
$endgroup$
add a comment |
$begingroup$
$cos^{-1}x+cos^{-1}y=cos^{-1}Big(xy-sqrt{1-x^2}sqrt{1-y^2}Big)$ true for all $x$ ?
My Attempt
Let $a=cos^{-1}x$, $b=cos^{-1}yimplies$$cos a=x$, $cos b=y$ and $a,bin[0,pi]implies a+bin[0,2pi]$
$$
cos(a+b)=cos acos b-sin asin b=xy-sqrt{1-x^2}sqrt{1-y^2}\=cosbigg[cos^{-1}Big(xy-sqrt{1-x^2}sqrt{1-y^2}Big)bigg]\
a+b=color{red}{cos^{-1}x+cos^{-1}y=2npipmcos^{-1}Big(xy-sqrt{1-x^2}sqrt{1-y^2}Big)}
$$
Case 1: $a+bin[0,pi)$
$$
cos^{-1}x+cos^{-1}y=cos^{-1}Big(xy-sqrt{1-x^2}sqrt{1-y^2}Big)
$$
Case 2: $a+bin[pi,2pi]$
$$
cos^{-1}x+cos^{-1}y=2pi-cos^{-1}Big(xy-sqrt{1-x^2}sqrt{1-y^2}Big)
$$
Case 1-: $a+bin[0,pi)$
Note I have checked a possible solution to this in link, but it gives a different expression differ from my attempt.
trigonometry inverse-function
$endgroup$
$begingroup$
See math.stackexchange.com/questions/3027787/…
$endgroup$
– lab bhattacharjee
Dec 6 '18 at 6:46
$begingroup$
@labbhattacharjee thanx. but the given post does not say anything about all cases for the expression nor the domain of $x,y$ for which the given expression is valid !. I hardly find any post on the complete expression for $cos^{-1}x+cos^{-1}y$ on internet.
$endgroup$
– ss1729
Dec 6 '18 at 6:47
$begingroup$
For $x=y=-1$, the LHS is $2pi$, the RHS is $0$.
$endgroup$
– Andrei
Dec 6 '18 at 6:53
$begingroup$
@Andrei For $x=y=-1implies a=b=pi$. RHS=$2pi-cos^{-1}1=2pi=LHS$, it is case 2. srry abt that, just edited the domain.
$endgroup$
– ss1729
Dec 6 '18 at 7:12
$begingroup$
@labbhattacharjee could you please comment on my attempt ?
$endgroup$
– ss1729
Dec 6 '18 at 7:16
add a comment |
$begingroup$
$cos^{-1}x+cos^{-1}y=cos^{-1}Big(xy-sqrt{1-x^2}sqrt{1-y^2}Big)$ true for all $x$ ?
My Attempt
Let $a=cos^{-1}x$, $b=cos^{-1}yimplies$$cos a=x$, $cos b=y$ and $a,bin[0,pi]implies a+bin[0,2pi]$
$$
cos(a+b)=cos acos b-sin asin b=xy-sqrt{1-x^2}sqrt{1-y^2}\=cosbigg[cos^{-1}Big(xy-sqrt{1-x^2}sqrt{1-y^2}Big)bigg]\
a+b=color{red}{cos^{-1}x+cos^{-1}y=2npipmcos^{-1}Big(xy-sqrt{1-x^2}sqrt{1-y^2}Big)}
$$
Case 1: $a+bin[0,pi)$
$$
cos^{-1}x+cos^{-1}y=cos^{-1}Big(xy-sqrt{1-x^2}sqrt{1-y^2}Big)
$$
Case 2: $a+bin[pi,2pi]$
$$
cos^{-1}x+cos^{-1}y=2pi-cos^{-1}Big(xy-sqrt{1-x^2}sqrt{1-y^2}Big)
$$
Case 1-: $a+bin[0,pi)$
Note I have checked a possible solution to this in link, but it gives a different expression differ from my attempt.
trigonometry inverse-function
$endgroup$
$cos^{-1}x+cos^{-1}y=cos^{-1}Big(xy-sqrt{1-x^2}sqrt{1-y^2}Big)$ true for all $x$ ?
My Attempt
Let $a=cos^{-1}x$, $b=cos^{-1}yimplies$$cos a=x$, $cos b=y$ and $a,bin[0,pi]implies a+bin[0,2pi]$
$$
cos(a+b)=cos acos b-sin asin b=xy-sqrt{1-x^2}sqrt{1-y^2}\=cosbigg[cos^{-1}Big(xy-sqrt{1-x^2}sqrt{1-y^2}Big)bigg]\
a+b=color{red}{cos^{-1}x+cos^{-1}y=2npipmcos^{-1}Big(xy-sqrt{1-x^2}sqrt{1-y^2}Big)}
$$
Case 1: $a+bin[0,pi)$
$$
cos^{-1}x+cos^{-1}y=cos^{-1}Big(xy-sqrt{1-x^2}sqrt{1-y^2}Big)
$$
Case 2: $a+bin[pi,2pi]$
$$
cos^{-1}x+cos^{-1}y=2pi-cos^{-1}Big(xy-sqrt{1-x^2}sqrt{1-y^2}Big)
$$
Case 1-: $a+bin[0,pi)$
Note I have checked a possible solution to this in link, but it gives a different expression differ from my attempt.
trigonometry inverse-function
trigonometry inverse-function
edited Dec 6 '18 at 7:13
ss1729
asked Dec 6 '18 at 6:45
ss1729ss1729
2,00111024
2,00111024
$begingroup$
See math.stackexchange.com/questions/3027787/…
$endgroup$
– lab bhattacharjee
Dec 6 '18 at 6:46
$begingroup$
@labbhattacharjee thanx. but the given post does not say anything about all cases for the expression nor the domain of $x,y$ for which the given expression is valid !. I hardly find any post on the complete expression for $cos^{-1}x+cos^{-1}y$ on internet.
$endgroup$
– ss1729
Dec 6 '18 at 6:47
$begingroup$
For $x=y=-1$, the LHS is $2pi$, the RHS is $0$.
$endgroup$
– Andrei
Dec 6 '18 at 6:53
$begingroup$
@Andrei For $x=y=-1implies a=b=pi$. RHS=$2pi-cos^{-1}1=2pi=LHS$, it is case 2. srry abt that, just edited the domain.
$endgroup$
– ss1729
Dec 6 '18 at 7:12
$begingroup$
@labbhattacharjee could you please comment on my attempt ?
$endgroup$
– ss1729
Dec 6 '18 at 7:16
add a comment |
$begingroup$
See math.stackexchange.com/questions/3027787/…
$endgroup$
– lab bhattacharjee
Dec 6 '18 at 6:46
$begingroup$
@labbhattacharjee thanx. but the given post does not say anything about all cases for the expression nor the domain of $x,y$ for which the given expression is valid !. I hardly find any post on the complete expression for $cos^{-1}x+cos^{-1}y$ on internet.
$endgroup$
– ss1729
Dec 6 '18 at 6:47
$begingroup$
For $x=y=-1$, the LHS is $2pi$, the RHS is $0$.
$endgroup$
– Andrei
Dec 6 '18 at 6:53
$begingroup$
@Andrei For $x=y=-1implies a=b=pi$. RHS=$2pi-cos^{-1}1=2pi=LHS$, it is case 2. srry abt that, just edited the domain.
$endgroup$
– ss1729
Dec 6 '18 at 7:12
$begingroup$
@labbhattacharjee could you please comment on my attempt ?
$endgroup$
– ss1729
Dec 6 '18 at 7:16
$begingroup$
See math.stackexchange.com/questions/3027787/…
$endgroup$
– lab bhattacharjee
Dec 6 '18 at 6:46
$begingroup$
See math.stackexchange.com/questions/3027787/…
$endgroup$
– lab bhattacharjee
Dec 6 '18 at 6:46
$begingroup$
@labbhattacharjee thanx. but the given post does not say anything about all cases for the expression nor the domain of $x,y$ for which the given expression is valid !. I hardly find any post on the complete expression for $cos^{-1}x+cos^{-1}y$ on internet.
$endgroup$
– ss1729
Dec 6 '18 at 6:47
$begingroup$
@labbhattacharjee thanx. but the given post does not say anything about all cases for the expression nor the domain of $x,y$ for which the given expression is valid !. I hardly find any post on the complete expression for $cos^{-1}x+cos^{-1}y$ on internet.
$endgroup$
– ss1729
Dec 6 '18 at 6:47
$begingroup$
For $x=y=-1$, the LHS is $2pi$, the RHS is $0$.
$endgroup$
– Andrei
Dec 6 '18 at 6:53
$begingroup$
For $x=y=-1$, the LHS is $2pi$, the RHS is $0$.
$endgroup$
– Andrei
Dec 6 '18 at 6:53
$begingroup$
@Andrei For $x=y=-1implies a=b=pi$. RHS=$2pi-cos^{-1}1=2pi=LHS$, it is case 2. srry abt that, just edited the domain.
$endgroup$
– ss1729
Dec 6 '18 at 7:12
$begingroup$
@Andrei For $x=y=-1implies a=b=pi$. RHS=$2pi-cos^{-1}1=2pi=LHS$, it is case 2. srry abt that, just edited the domain.
$endgroup$
– ss1729
Dec 6 '18 at 7:12
$begingroup$
@labbhattacharjee could you please comment on my attempt ?
$endgroup$
– ss1729
Dec 6 '18 at 7:16
$begingroup$
@labbhattacharjee could you please comment on my attempt ?
$endgroup$
– ss1729
Dec 6 '18 at 7:16
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
Using Principal values and Why it's true? $arcsin(x) +arccos(x) = frac{pi}{2}$,
$cos^{-1}x+cos^{-1}yle2pi$
$$cos^{-1}x+cos^{-1}y=cos^{-1}(xy-sqrt{(1-x^2)(1-y^2)})$$
will hold true if $cos^{-1}x+cos^{-1}ylepi$
$iffsin^{-1}x+sin^{-1}yge0iffsin^{-1}xge-sin^{-1}y=sin^{-1}(-y)$
$iff xge -yiff x+yge0$
$$cos^{-1}x+cos^{-1}y=2pi-cos^{-1}(xy-sqrt{(1-x^2)(1-y^2)})$$
will hold true if $cos^{-1}x+cos^{-1}y>pi$
$iffsin^{-1}x+sin^{-1}y<0iff x+y<0$
$endgroup$
$begingroup$
Thanks. I just have one more query. Could u please confirm that, $cos^{-1}x-cos^{-1}y=begin{cases}cos^{-1}bigg(xy+sqrt{1-x^2}sqrt{1-y^2}bigg),;x-yleq 0\ -2pi+cos^{-1}bigg(xy+sqrt{1-x^2}sqrt{1-y^2}bigg),;x-y> 0end{cases}$
$endgroup$
– ss1729
Dec 6 '18 at 17:48
1
$begingroup$
@ss1729, First replace $y$ with $-z$ in the identity derived in the answer. Then use math.stackexchange.com/questions/1224415/…
$endgroup$
– lab bhattacharjee
Dec 6 '18 at 18:24
add a comment |
Your Answer
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Using Principal values and Why it's true? $arcsin(x) +arccos(x) = frac{pi}{2}$,
$cos^{-1}x+cos^{-1}yle2pi$
$$cos^{-1}x+cos^{-1}y=cos^{-1}(xy-sqrt{(1-x^2)(1-y^2)})$$
will hold true if $cos^{-1}x+cos^{-1}ylepi$
$iffsin^{-1}x+sin^{-1}yge0iffsin^{-1}xge-sin^{-1}y=sin^{-1}(-y)$
$iff xge -yiff x+yge0$
$$cos^{-1}x+cos^{-1}y=2pi-cos^{-1}(xy-sqrt{(1-x^2)(1-y^2)})$$
will hold true if $cos^{-1}x+cos^{-1}y>pi$
$iffsin^{-1}x+sin^{-1}y<0iff x+y<0$
$endgroup$
$begingroup$
Thanks. I just have one more query. Could u please confirm that, $cos^{-1}x-cos^{-1}y=begin{cases}cos^{-1}bigg(xy+sqrt{1-x^2}sqrt{1-y^2}bigg),;x-yleq 0\ -2pi+cos^{-1}bigg(xy+sqrt{1-x^2}sqrt{1-y^2}bigg),;x-y> 0end{cases}$
$endgroup$
– ss1729
Dec 6 '18 at 17:48
1
$begingroup$
@ss1729, First replace $y$ with $-z$ in the identity derived in the answer. Then use math.stackexchange.com/questions/1224415/…
$endgroup$
– lab bhattacharjee
Dec 6 '18 at 18:24
add a comment |
$begingroup$
Using Principal values and Why it's true? $arcsin(x) +arccos(x) = frac{pi}{2}$,
$cos^{-1}x+cos^{-1}yle2pi$
$$cos^{-1}x+cos^{-1}y=cos^{-1}(xy-sqrt{(1-x^2)(1-y^2)})$$
will hold true if $cos^{-1}x+cos^{-1}ylepi$
$iffsin^{-1}x+sin^{-1}yge0iffsin^{-1}xge-sin^{-1}y=sin^{-1}(-y)$
$iff xge -yiff x+yge0$
$$cos^{-1}x+cos^{-1}y=2pi-cos^{-1}(xy-sqrt{(1-x^2)(1-y^2)})$$
will hold true if $cos^{-1}x+cos^{-1}y>pi$
$iffsin^{-1}x+sin^{-1}y<0iff x+y<0$
$endgroup$
$begingroup$
Thanks. I just have one more query. Could u please confirm that, $cos^{-1}x-cos^{-1}y=begin{cases}cos^{-1}bigg(xy+sqrt{1-x^2}sqrt{1-y^2}bigg),;x-yleq 0\ -2pi+cos^{-1}bigg(xy+sqrt{1-x^2}sqrt{1-y^2}bigg),;x-y> 0end{cases}$
$endgroup$
– ss1729
Dec 6 '18 at 17:48
1
$begingroup$
@ss1729, First replace $y$ with $-z$ in the identity derived in the answer. Then use math.stackexchange.com/questions/1224415/…
$endgroup$
– lab bhattacharjee
Dec 6 '18 at 18:24
add a comment |
$begingroup$
Using Principal values and Why it's true? $arcsin(x) +arccos(x) = frac{pi}{2}$,
$cos^{-1}x+cos^{-1}yle2pi$
$$cos^{-1}x+cos^{-1}y=cos^{-1}(xy-sqrt{(1-x^2)(1-y^2)})$$
will hold true if $cos^{-1}x+cos^{-1}ylepi$
$iffsin^{-1}x+sin^{-1}yge0iffsin^{-1}xge-sin^{-1}y=sin^{-1}(-y)$
$iff xge -yiff x+yge0$
$$cos^{-1}x+cos^{-1}y=2pi-cos^{-1}(xy-sqrt{(1-x^2)(1-y^2)})$$
will hold true if $cos^{-1}x+cos^{-1}y>pi$
$iffsin^{-1}x+sin^{-1}y<0iff x+y<0$
$endgroup$
Using Principal values and Why it's true? $arcsin(x) +arccos(x) = frac{pi}{2}$,
$cos^{-1}x+cos^{-1}yle2pi$
$$cos^{-1}x+cos^{-1}y=cos^{-1}(xy-sqrt{(1-x^2)(1-y^2)})$$
will hold true if $cos^{-1}x+cos^{-1}ylepi$
$iffsin^{-1}x+sin^{-1}yge0iffsin^{-1}xge-sin^{-1}y=sin^{-1}(-y)$
$iff xge -yiff x+yge0$
$$cos^{-1}x+cos^{-1}y=2pi-cos^{-1}(xy-sqrt{(1-x^2)(1-y^2)})$$
will hold true if $cos^{-1}x+cos^{-1}y>pi$
$iffsin^{-1}x+sin^{-1}y<0iff x+y<0$
answered Dec 6 '18 at 7:33
lab bhattacharjeelab bhattacharjee
226k15157275
226k15157275
$begingroup$
Thanks. I just have one more query. Could u please confirm that, $cos^{-1}x-cos^{-1}y=begin{cases}cos^{-1}bigg(xy+sqrt{1-x^2}sqrt{1-y^2}bigg),;x-yleq 0\ -2pi+cos^{-1}bigg(xy+sqrt{1-x^2}sqrt{1-y^2}bigg),;x-y> 0end{cases}$
$endgroup$
– ss1729
Dec 6 '18 at 17:48
1
$begingroup$
@ss1729, First replace $y$ with $-z$ in the identity derived in the answer. Then use math.stackexchange.com/questions/1224415/…
$endgroup$
– lab bhattacharjee
Dec 6 '18 at 18:24
add a comment |
$begingroup$
Thanks. I just have one more query. Could u please confirm that, $cos^{-1}x-cos^{-1}y=begin{cases}cos^{-1}bigg(xy+sqrt{1-x^2}sqrt{1-y^2}bigg),;x-yleq 0\ -2pi+cos^{-1}bigg(xy+sqrt{1-x^2}sqrt{1-y^2}bigg),;x-y> 0end{cases}$
$endgroup$
– ss1729
Dec 6 '18 at 17:48
1
$begingroup$
@ss1729, First replace $y$ with $-z$ in the identity derived in the answer. Then use math.stackexchange.com/questions/1224415/…
$endgroup$
– lab bhattacharjee
Dec 6 '18 at 18:24
$begingroup$
Thanks. I just have one more query. Could u please confirm that, $cos^{-1}x-cos^{-1}y=begin{cases}cos^{-1}bigg(xy+sqrt{1-x^2}sqrt{1-y^2}bigg),;x-yleq 0\ -2pi+cos^{-1}bigg(xy+sqrt{1-x^2}sqrt{1-y^2}bigg),;x-y> 0end{cases}$
$endgroup$
– ss1729
Dec 6 '18 at 17:48
$begingroup$
Thanks. I just have one more query. Could u please confirm that, $cos^{-1}x-cos^{-1}y=begin{cases}cos^{-1}bigg(xy+sqrt{1-x^2}sqrt{1-y^2}bigg),;x-yleq 0\ -2pi+cos^{-1}bigg(xy+sqrt{1-x^2}sqrt{1-y^2}bigg),;x-y> 0end{cases}$
$endgroup$
– ss1729
Dec 6 '18 at 17:48
1
1
$begingroup$
@ss1729, First replace $y$ with $-z$ in the identity derived in the answer. Then use math.stackexchange.com/questions/1224415/…
$endgroup$
– lab bhattacharjee
Dec 6 '18 at 18:24
$begingroup$
@ss1729, First replace $y$ with $-z$ in the identity derived in the answer. Then use math.stackexchange.com/questions/1224415/…
$endgroup$
– lab bhattacharjee
Dec 6 '18 at 18:24
add a comment |
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$begingroup$
See math.stackexchange.com/questions/3027787/…
$endgroup$
– lab bhattacharjee
Dec 6 '18 at 6:46
$begingroup$
@labbhattacharjee thanx. but the given post does not say anything about all cases for the expression nor the domain of $x,y$ for which the given expression is valid !. I hardly find any post on the complete expression for $cos^{-1}x+cos^{-1}y$ on internet.
$endgroup$
– ss1729
Dec 6 '18 at 6:47
$begingroup$
For $x=y=-1$, the LHS is $2pi$, the RHS is $0$.
$endgroup$
– Andrei
Dec 6 '18 at 6:53
$begingroup$
@Andrei For $x=y=-1implies a=b=pi$. RHS=$2pi-cos^{-1}1=2pi=LHS$, it is case 2. srry abt that, just edited the domain.
$endgroup$
– ss1729
Dec 6 '18 at 7:12
$begingroup$
@labbhattacharjee could you please comment on my attempt ?
$endgroup$
– ss1729
Dec 6 '18 at 7:16