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hlbeckley
Topic Author
Posts: 15
Joined: July 1st, 2009, 7:18 am

### Differential Problem

Hi all, please help me.I have the question:f(x,y) = x^2 + y^2, where x=sin2theta and y=cos2thetaWhat is df/dtheta?I just dont know where to begin pls can some one give guidance to me? I want to try understand the porblem, not simply just get an answerthanks

ACD
Posts: 107
Joined: April 19th, 2004, 8:09 am

### Differential Problem

Look up the chain rule

daveangel
Posts: 17031
Joined: October 20th, 2003, 4:05 pm

### Differential Problem

are you kidding ?
knowledge comes, wisdom lingers

hlbeckley
Topic Author
Posts: 15
Joined: July 1st, 2009, 7:18 am

### Differential Problem

I kid you not!

daveangel
Posts: 17031
Joined: October 20th, 2003, 4:05 pm

### Differential Problem

hint: for any x
knowledge comes, wisdom lingers

hlbeckley
Topic Author
Posts: 15
Joined: July 1st, 2009, 7:18 am

### Differential Problem

Ok sure get that for A-Level dayshere's my mind thinking...pls hold the laughterx = sin2theta thus x^2 = sin^2.2thetay = cos2theta thus y^2 = cos^2.2thetaso from Q; f(x,y) = x^2 + y^2f(x,y) = sin^2.2theta + cos^2.2thetaf(x,y) = 2theta(sin^2+cos^2) = 2theta(1)df/dtheta is = 2???

hlbeckley
Topic Author
Posts: 15
Joined: July 1st, 2009, 7:18 am

### Differential Problem

hang on my factorising is offsin^2.2theta + cos^2.2theta = 1so f(x,y) now = 1and the f' of 1 = 0

batesh
Posts: 31
Joined: December 9th, 2005, 12:29 pm

### Differential Problem

Without using the trig identity you can go down the total derivative route to arrive at the same answer.

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