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-   -   Proving using calculus without trig identity (http://www.physicsforums.com/showthread.php?t=580483)

 kebabs Feb23-12 01:35 AM

proving using calculus without trig identity

Please I really need help with this hw question

Prove without trig identity that f(x)=0 for

F(x)=Asin^2(Bx+C)+Acos^2(Bx+C)

 SteveL27 Feb23-12 01:47 AM

Re: proving using calculus without trig identity

Quote:
 Quote by kebabs (Post 3779441) Please I really need help with this hw question Prove without trig identity that f(x)=0 for F(x)=Asin^2(Bx+C)+Acos^2(Bx+C)
You're not supposed to use the obvious identity that simplifies this? I suppose you could just use the derivatives of sin and cos along with the chain rule to directly compute the derivative. But eventually you'll need to simplify using some trig identity.

 kebabs Feb23-12 01:48 AM

Re: proving using calculus without trig identity

I can't use trig identy to solve it

 kebabs Feb23-12 01:50 AM

Re: proving using calculus without trig identity

I mean I'm not allowed to

 SammyS Feb23-12 03:03 PM

Re: proving using calculus without trig identity

Quote:
 Quote by kebabs (Post 3779441) Please I really need help with this hw question Prove without trig identity that f`(x)=0 for F(x)=Asin^2(Bx+C)+Acos^2(Bx+C)
What is F'(x) if $F(x)=A\sin^2(Bx+C)+A\cos^2(Bx+C)\,?$

 eumyang Feb23-12 03:22 PM

Re: proving using calculus without trig identity

Quote:
 Quote by SteveL27 (Post 3779455) You're not supposed to use the obvious identity that simplifies this? I suppose you could just use the derivatives of sin and cos along with the chain rule to directly compute the derivative. But eventually you'll need to simplify using some trig identity.
Are you sure? I was able to get F'(x) = 0 by using the chain rule, and yet I didn't use any trig identity.

 kebabs Feb26-12 05:22 PM

Re: proving using calculus without trig identity

 Ansatz7 Feb26-12 05:30 PM

Re: proving using calculus without trig identity

It's really simple - just use chain rule to take the d/dx of the whole expression. No trig or any other kinds of tricks necessary. Are you familiar with the use of chain rule?

 Mark44 Feb26-12 05:53 PM

Re: proving using calculus without trig identity

Quote:
 Quote by kebabs (Post 3785858) could you please send me your working for this question??
This is not permitted at Physics Forums - don't even ask.

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