Resolution exercises on complex numbers
How do I solve this equation in the field of complex numbers?:
$$|z|^2 - z|z| + z = 0 $$
My solutions are:
$$z_1 = 0$$
$$z_2 = -1$$
complex-numbers
add a comment |
How do I solve this equation in the field of complex numbers?:
$$|z|^2 - z|z| + z = 0 $$
My solutions are:
$$z_1 = 0$$
$$z_2 = -1$$
complex-numbers
1
$-1$ is not a solution as you would get $1+1-1not =0$
– Henry
Nov 21 '18 at 11:20
add a comment |
How do I solve this equation in the field of complex numbers?:
$$|z|^2 - z|z| + z = 0 $$
My solutions are:
$$z_1 = 0$$
$$z_2 = -1$$
complex-numbers
How do I solve this equation in the field of complex numbers?:
$$|z|^2 - z|z| + z = 0 $$
My solutions are:
$$z_1 = 0$$
$$z_2 = -1$$
complex-numbers
complex-numbers
edited Nov 21 '18 at 7:41
asked Nov 21 '18 at 7:35
Vincenzo Iannucci
343
343
1
$-1$ is not a solution as you would get $1+1-1not =0$
– Henry
Nov 21 '18 at 11:20
add a comment |
1
$-1$ is not a solution as you would get $1+1-1not =0$
– Henry
Nov 21 '18 at 11:20
1
1
$-1$ is not a solution as you would get $1+1-1not =0$
– Henry
Nov 21 '18 at 11:20
$-1$ is not a solution as you would get $1+1-1not =0$
– Henry
Nov 21 '18 at 11:20
add a comment |
5 Answers
5
active
oldest
votes
$$z=dfrac{|z|^2}{|z|-1}$$ which is real
If $z>0,|z|=+z$ $$0=z^2-z^2+ziff z=0$$ which is untenable
If $zle0,|z|=-z$ $$0=z^2+z^2+z=z(2z+1)$$
Your first step only works when $|z| ne 1$, so I think you need to explicitly/separately rule out the case that $|z| = 1$. (This isn't too difficult, but I think it's important, if only because the OP believes that $z = -1$ is a solution.)
– ruakh
Nov 21 '18 at 23:29
add a comment |
Observe that $z=0$ is a solution of the equation
$$|z|^2 - z|z| + z = 0.$$
( $z=-1$ is not a solution !)
Therefore let $z ne 0$ be a further solution of this equation. We get , since $|z|^2=z overline{z}:$
$overline{z}-|z|+1=0$. This gives $overline{z}=|z|-1 in mathbb R$.Hence
$z=|z|-1 in mathbb R$.
If $z>0$ , we have $z=z-1$, which is impossible. Hence $z<0$ and then $z=-1/2$.
add a comment |
WLOG $z=r(cos t+isin t)$ where $r>0,t$ are real
$$r(r-r(cos t+isin t)+(cos t+isin t))=0$$
If $rne0,$ equating the real & the imaginary parts
$r-rcos t+cos t=0=(1-r)sin t$
Case $#1:$
If $r=1,1=0$ which is untenable
If $sin t=0,$
Case $#2A:cos t=1,r=0$ which is untenable
Case $#2A:cos t=-1,r+r-1=0iff r=?$
add a comment |
You may also proceed as follows:
- Rewrite the equation to
$$|z|^2 - z|z| + z = 0 Leftrightarrow boxed{|z|^2 = z(|z|-1)}$$
- Therefore, $color{blue}{z}$ must be $color{blue}{mbox{real}}$ and so we have $color{blue}{|z|^2 = z^2}$.
- Noting the solution $boxed{z = 0}$ we get
$$|z|^2 = z(|z|-1) stackrel{z in mathbb{R}, z neq 0}{Leftrightarrow}z = |z|-1 Rightarrow boxed{z = -frac{1}{2}}$$
add a comment |
Let $r = e^{itheta}$.
We get $r^2 - re^{itheta}cdot r + re^{itheta} = 0$.
Factorising, we get $r = 0$, giving $z = 0$ as one solution or:
$r - re^{itheta} + e^{itheta} = 0$
$e^{itheta} = frac{r}{r-1}$
From the last equation, the magnitude of the LHS is equal to one. From the RHS, $e^{itheta} in mathbb{R}$. Hence $e^{itheta} = pm 1$.
$frac{r}{r-1} = 1$ gives no solution, but $frac{r}{r-1} = -1 implies r = frac 12$. Since $e^{itheta} = -1$ that gives $z = -frac 12$.
Therefore the two solutions are $z = 0$ and $z = -frac 12$.
add a comment |
Your Answer
StackExchange.ifUsing("editor", function () {
return StackExchange.using("mathjaxEditing", function () {
StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
});
});
}, "mathjax-editing");
StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "69"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});
function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
noCode: true, onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3007387%2fresolution-exercises-on-complex-numbers%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
5 Answers
5
active
oldest
votes
5 Answers
5
active
oldest
votes
active
oldest
votes
active
oldest
votes
$$z=dfrac{|z|^2}{|z|-1}$$ which is real
If $z>0,|z|=+z$ $$0=z^2-z^2+ziff z=0$$ which is untenable
If $zle0,|z|=-z$ $$0=z^2+z^2+z=z(2z+1)$$
Your first step only works when $|z| ne 1$, so I think you need to explicitly/separately rule out the case that $|z| = 1$. (This isn't too difficult, but I think it's important, if only because the OP believes that $z = -1$ is a solution.)
– ruakh
Nov 21 '18 at 23:29
add a comment |
$$z=dfrac{|z|^2}{|z|-1}$$ which is real
If $z>0,|z|=+z$ $$0=z^2-z^2+ziff z=0$$ which is untenable
If $zle0,|z|=-z$ $$0=z^2+z^2+z=z(2z+1)$$
Your first step only works when $|z| ne 1$, so I think you need to explicitly/separately rule out the case that $|z| = 1$. (This isn't too difficult, but I think it's important, if only because the OP believes that $z = -1$ is a solution.)
– ruakh
Nov 21 '18 at 23:29
add a comment |
$$z=dfrac{|z|^2}{|z|-1}$$ which is real
If $z>0,|z|=+z$ $$0=z^2-z^2+ziff z=0$$ which is untenable
If $zle0,|z|=-z$ $$0=z^2+z^2+z=z(2z+1)$$
$$z=dfrac{|z|^2}{|z|-1}$$ which is real
If $z>0,|z|=+z$ $$0=z^2-z^2+ziff z=0$$ which is untenable
If $zle0,|z|=-z$ $$0=z^2+z^2+z=z(2z+1)$$
edited Nov 21 '18 at 13:51
answered Nov 21 '18 at 7:58
lab bhattacharjee
223k15156274
223k15156274
Your first step only works when $|z| ne 1$, so I think you need to explicitly/separately rule out the case that $|z| = 1$. (This isn't too difficult, but I think it's important, if only because the OP believes that $z = -1$ is a solution.)
– ruakh
Nov 21 '18 at 23:29
add a comment |
Your first step only works when $|z| ne 1$, so I think you need to explicitly/separately rule out the case that $|z| = 1$. (This isn't too difficult, but I think it's important, if only because the OP believes that $z = -1$ is a solution.)
– ruakh
Nov 21 '18 at 23:29
Your first step only works when $|z| ne 1$, so I think you need to explicitly/separately rule out the case that $|z| = 1$. (This isn't too difficult, but I think it's important, if only because the OP believes that $z = -1$ is a solution.)
– ruakh
Nov 21 '18 at 23:29
Your first step only works when $|z| ne 1$, so I think you need to explicitly/separately rule out the case that $|z| = 1$. (This isn't too difficult, but I think it's important, if only because the OP believes that $z = -1$ is a solution.)
– ruakh
Nov 21 '18 at 23:29
add a comment |
Observe that $z=0$ is a solution of the equation
$$|z|^2 - z|z| + z = 0.$$
( $z=-1$ is not a solution !)
Therefore let $z ne 0$ be a further solution of this equation. We get , since $|z|^2=z overline{z}:$
$overline{z}-|z|+1=0$. This gives $overline{z}=|z|-1 in mathbb R$.Hence
$z=|z|-1 in mathbb R$.
If $z>0$ , we have $z=z-1$, which is impossible. Hence $z<0$ and then $z=-1/2$.
add a comment |
Observe that $z=0$ is a solution of the equation
$$|z|^2 - z|z| + z = 0.$$
( $z=-1$ is not a solution !)
Therefore let $z ne 0$ be a further solution of this equation. We get , since $|z|^2=z overline{z}:$
$overline{z}-|z|+1=0$. This gives $overline{z}=|z|-1 in mathbb R$.Hence
$z=|z|-1 in mathbb R$.
If $z>0$ , we have $z=z-1$, which is impossible. Hence $z<0$ and then $z=-1/2$.
add a comment |
Observe that $z=0$ is a solution of the equation
$$|z|^2 - z|z| + z = 0.$$
( $z=-1$ is not a solution !)
Therefore let $z ne 0$ be a further solution of this equation. We get , since $|z|^2=z overline{z}:$
$overline{z}-|z|+1=0$. This gives $overline{z}=|z|-1 in mathbb R$.Hence
$z=|z|-1 in mathbb R$.
If $z>0$ , we have $z=z-1$, which is impossible. Hence $z<0$ and then $z=-1/2$.
Observe that $z=0$ is a solution of the equation
$$|z|^2 - z|z| + z = 0.$$
( $z=-1$ is not a solution !)
Therefore let $z ne 0$ be a further solution of this equation. We get , since $|z|^2=z overline{z}:$
$overline{z}-|z|+1=0$. This gives $overline{z}=|z|-1 in mathbb R$.Hence
$z=|z|-1 in mathbb R$.
If $z>0$ , we have $z=z-1$, which is impossible. Hence $z<0$ and then $z=-1/2$.
answered Nov 21 '18 at 7:42
Fred
44.2k1845
44.2k1845
add a comment |
add a comment |
WLOG $z=r(cos t+isin t)$ where $r>0,t$ are real
$$r(r-r(cos t+isin t)+(cos t+isin t))=0$$
If $rne0,$ equating the real & the imaginary parts
$r-rcos t+cos t=0=(1-r)sin t$
Case $#1:$
If $r=1,1=0$ which is untenable
If $sin t=0,$
Case $#2A:cos t=1,r=0$ which is untenable
Case $#2A:cos t=-1,r+r-1=0iff r=?$
add a comment |
WLOG $z=r(cos t+isin t)$ where $r>0,t$ are real
$$r(r-r(cos t+isin t)+(cos t+isin t))=0$$
If $rne0,$ equating the real & the imaginary parts
$r-rcos t+cos t=0=(1-r)sin t$
Case $#1:$
If $r=1,1=0$ which is untenable
If $sin t=0,$
Case $#2A:cos t=1,r=0$ which is untenable
Case $#2A:cos t=-1,r+r-1=0iff r=?$
add a comment |
WLOG $z=r(cos t+isin t)$ where $r>0,t$ are real
$$r(r-r(cos t+isin t)+(cos t+isin t))=0$$
If $rne0,$ equating the real & the imaginary parts
$r-rcos t+cos t=0=(1-r)sin t$
Case $#1:$
If $r=1,1=0$ which is untenable
If $sin t=0,$
Case $#2A:cos t=1,r=0$ which is untenable
Case $#2A:cos t=-1,r+r-1=0iff r=?$
WLOG $z=r(cos t+isin t)$ where $r>0,t$ are real
$$r(r-r(cos t+isin t)+(cos t+isin t))=0$$
If $rne0,$ equating the real & the imaginary parts
$r-rcos t+cos t=0=(1-r)sin t$
Case $#1:$
If $r=1,1=0$ which is untenable
If $sin t=0,$
Case $#2A:cos t=1,r=0$ which is untenable
Case $#2A:cos t=-1,r+r-1=0iff r=?$
answered Nov 21 '18 at 7:42
lab bhattacharjee
223k15156274
223k15156274
add a comment |
add a comment |
You may also proceed as follows:
- Rewrite the equation to
$$|z|^2 - z|z| + z = 0 Leftrightarrow boxed{|z|^2 = z(|z|-1)}$$
- Therefore, $color{blue}{z}$ must be $color{blue}{mbox{real}}$ and so we have $color{blue}{|z|^2 = z^2}$.
- Noting the solution $boxed{z = 0}$ we get
$$|z|^2 = z(|z|-1) stackrel{z in mathbb{R}, z neq 0}{Leftrightarrow}z = |z|-1 Rightarrow boxed{z = -frac{1}{2}}$$
add a comment |
You may also proceed as follows:
- Rewrite the equation to
$$|z|^2 - z|z| + z = 0 Leftrightarrow boxed{|z|^2 = z(|z|-1)}$$
- Therefore, $color{blue}{z}$ must be $color{blue}{mbox{real}}$ and so we have $color{blue}{|z|^2 = z^2}$.
- Noting the solution $boxed{z = 0}$ we get
$$|z|^2 = z(|z|-1) stackrel{z in mathbb{R}, z neq 0}{Leftrightarrow}z = |z|-1 Rightarrow boxed{z = -frac{1}{2}}$$
add a comment |
You may also proceed as follows:
- Rewrite the equation to
$$|z|^2 - z|z| + z = 0 Leftrightarrow boxed{|z|^2 = z(|z|-1)}$$
- Therefore, $color{blue}{z}$ must be $color{blue}{mbox{real}}$ and so we have $color{blue}{|z|^2 = z^2}$.
- Noting the solution $boxed{z = 0}$ we get
$$|z|^2 = z(|z|-1) stackrel{z in mathbb{R}, z neq 0}{Leftrightarrow}z = |z|-1 Rightarrow boxed{z = -frac{1}{2}}$$
You may also proceed as follows:
- Rewrite the equation to
$$|z|^2 - z|z| + z = 0 Leftrightarrow boxed{|z|^2 = z(|z|-1)}$$
- Therefore, $color{blue}{z}$ must be $color{blue}{mbox{real}}$ and so we have $color{blue}{|z|^2 = z^2}$.
- Noting the solution $boxed{z = 0}$ we get
$$|z|^2 = z(|z|-1) stackrel{z in mathbb{R}, z neq 0}{Leftrightarrow}z = |z|-1 Rightarrow boxed{z = -frac{1}{2}}$$
edited Nov 21 '18 at 15:38
answered Nov 21 '18 at 8:29
trancelocation
9,1051521
9,1051521
add a comment |
add a comment |
Let $r = e^{itheta}$.
We get $r^2 - re^{itheta}cdot r + re^{itheta} = 0$.
Factorising, we get $r = 0$, giving $z = 0$ as one solution or:
$r - re^{itheta} + e^{itheta} = 0$
$e^{itheta} = frac{r}{r-1}$
From the last equation, the magnitude of the LHS is equal to one. From the RHS, $e^{itheta} in mathbb{R}$. Hence $e^{itheta} = pm 1$.
$frac{r}{r-1} = 1$ gives no solution, but $frac{r}{r-1} = -1 implies r = frac 12$. Since $e^{itheta} = -1$ that gives $z = -frac 12$.
Therefore the two solutions are $z = 0$ and $z = -frac 12$.
add a comment |
Let $r = e^{itheta}$.
We get $r^2 - re^{itheta}cdot r + re^{itheta} = 0$.
Factorising, we get $r = 0$, giving $z = 0$ as one solution or:
$r - re^{itheta} + e^{itheta} = 0$
$e^{itheta} = frac{r}{r-1}$
From the last equation, the magnitude of the LHS is equal to one. From the RHS, $e^{itheta} in mathbb{R}$. Hence $e^{itheta} = pm 1$.
$frac{r}{r-1} = 1$ gives no solution, but $frac{r}{r-1} = -1 implies r = frac 12$. Since $e^{itheta} = -1$ that gives $z = -frac 12$.
Therefore the two solutions are $z = 0$ and $z = -frac 12$.
add a comment |
Let $r = e^{itheta}$.
We get $r^2 - re^{itheta}cdot r + re^{itheta} = 0$.
Factorising, we get $r = 0$, giving $z = 0$ as one solution or:
$r - re^{itheta} + e^{itheta} = 0$
$e^{itheta} = frac{r}{r-1}$
From the last equation, the magnitude of the LHS is equal to one. From the RHS, $e^{itheta} in mathbb{R}$. Hence $e^{itheta} = pm 1$.
$frac{r}{r-1} = 1$ gives no solution, but $frac{r}{r-1} = -1 implies r = frac 12$. Since $e^{itheta} = -1$ that gives $z = -frac 12$.
Therefore the two solutions are $z = 0$ and $z = -frac 12$.
Let $r = e^{itheta}$.
We get $r^2 - re^{itheta}cdot r + re^{itheta} = 0$.
Factorising, we get $r = 0$, giving $z = 0$ as one solution or:
$r - re^{itheta} + e^{itheta} = 0$
$e^{itheta} = frac{r}{r-1}$
From the last equation, the magnitude of the LHS is equal to one. From the RHS, $e^{itheta} in mathbb{R}$. Hence $e^{itheta} = pm 1$.
$frac{r}{r-1} = 1$ gives no solution, but $frac{r}{r-1} = -1 implies r = frac 12$. Since $e^{itheta} = -1$ that gives $z = -frac 12$.
Therefore the two solutions are $z = 0$ and $z = -frac 12$.
answered Nov 21 '18 at 8:58
Deepak
16.8k11436
16.8k11436
add a comment |
add a comment |
Thanks for contributing an answer to Mathematics Stack Exchange!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
Use MathJax to format equations. MathJax reference.
To learn more, see our tips on writing great answers.
Some of your past answers have not been well-received, and you're in danger of being blocked from answering.
Please pay close attention to the following guidance:
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3007387%2fresolution-exercises-on-complex-numbers%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
1
$-1$ is not a solution as you would get $1+1-1not =0$
– Henry
Nov 21 '18 at 11:20