# $(m-n\cos y)(m+n\cos x)=m^2 -n^2$ then find $\frac{dy}{dx}$ [closed]

$(m-n\cos y)(m+n\cos x)=m^2 -n^2$ then find $\frac{dy}{dx}$

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use implicit differentiation and product rule. –  Alex Oct 11 '12 at 7:35
Alex can you give me the steps b'cause i have tried it many times but don't know today i am not getting the answer. plz –  sonu Oct 11 '12 at 7:38
Hi, Welcome to math.SE.Asking questions is fine, though I can't understand why you need to give unnecessary details about your class,etc. Also you wouldn't have asked for solution if you were not stuck. Please consider changing your title. –  TheJoker Oct 11 '12 at 7:44
no man this is not my home work these question is asked in my exam but i did'nt get answer so i need solution so if you can give reply then good ...thankyou in advance for giving me solution TheJoker Alex –  sonu Oct 11 '12 at 7:51
in advance? I have already answered... –  Alex Oct 11 '12 at 7:53

## 4 Answers

Suppose that you have a relation between $y$ and $x$, say $F(x,y)=0$ such that you can always find $y$ as a function of $x$. Then it can be proved that: $$\frac{dy}{dx}=\frac{-F_x}{F_y}$$ wherein $F_x$ means the derivative of $F$ regarding to variable $x$. Here put $m^2 -n^2=a$ as a constant and so $a'=0$. We have: $$F(x,y)=(m-n\cos y)(m+n\cos x)-a=0$$ then $$F(x,y)=m^2+mn\cos x-mn\cos y-n^2\cos x\cos y-a=0$$ then $$F_x(x,y)=-mn\sin x+n^2\sin x\cos y$$ and $$F_y(x,y)=mn\sin y+n^2\cos x\sin y$$

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plz i need the step i have tried all this stuff guys plz give me steps –  sonu Oct 11 '12 at 7:43
thnks Babak Sorouh –  sonu Oct 11 '12 at 7:57
what is your email id –  sonu Oct 11 '12 at 7:57
@sonu: You didn't even register here on MSE for a real person. Please do that. There are many like Alex and me for helping each other in Maths. No need any other job. :) –  Babak S. Oct 11 '12 at 8:07
@BabakSorouh agreed ;) –  Alex Oct 11 '12 at 8:10

$$(m-n\cos y)(m+n\cos x)=m^2 -n^2$$ $$(m-n\cos y)(-n\sin x) + (n\sin y)\bigg(\frac{dy}{dx}\bigg)(m+n\cos x)=0$$ $$\frac{dy}{dx}=\frac{(m-n\cos y)(n\sin x)}{(n\sin y)(m+n\cos x)}$$ $$\frac{dy}{dx}=\frac{(m-n\cos y)(\sin x)}{(\sin y)(m+n\cos x)}$$

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options are given in examples are –  sonu Oct 11 '12 at 7:53
A) m^2-n^2/m+ncosx b) root(m^2-n^2)/m+ncosx –  sonu Oct 11 '12 at 7:54
ans is 2 which i didn't get –  sonu Oct 11 '12 at 7:54
@sonu: Think about the answers (+1). –  Babak S. Oct 11 '12 at 8:09
@sonu: I think it would be good if you included the options into the text of your question. –  Martin Sleziak Oct 11 '12 at 8:11

@Alex has already showed you the steps. Use implicit differentiation and you'll get dy/dx, an then just solve for dy/dx.

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From $(m-n\cos y)(m+n \cos x)=m^2-n^2$ you can get $\cos y$. $$n(m+n\cos x)\cos y= n^2+mn\cos x\\ (m+n\cos x)\cos y = n+m\cos x\\ \cos y = \frac{n+m\cos x}{m+n\cos x}\\ y(x)=\arccos \left(\frac{n+m\cos x}{m+n\cos x}\right)$$ Then rest is an exercise in computing derivative. (Rather tedious, unless you notice some trick. I did not notice an easy way to do this.)

Here's what WA says. WolframAlpha gives $\csc(x) \sqrt{\frac{(m^2-n^2)\sin^2(x)}{(m+n\cos x)^2}}$ as a result, which is $\pm \frac{\sqrt{m^2-n^2}}{m+n\cos x}$. It seems close to one of your options.

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