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# FP2 - polar question help

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Why bother with a post grad course - waste of time? 17-10-2016
1. Hi, I can't find a solution bank for the Edecel FP2 exam style paper so though i'd ask here.

Here's the question:

I set x = rcos(theta)

then plug in r and differentiate, setting the differential equal to 0. But I cant get the expression in terms of just sin or cos.

using various Trig identities I've ended up with:

1/2 = (sin(theta))^2 + (sq/(3))(sin(theta)cos(theta))

help!
2. (Original post by Sir_Malc)
Hi, I can't find a solution bank for the Edecel FP2 exam style paper so though i'd ask here.

Here's the question:

I set x = rcos(theta)

then plug in r and differentiate, setting the differential equal to 0. But I cant get the expression in terms of just sin or cos.

using various Trig identities I've ended up with:

1/2 = (sin(theta))^2 + (sq/(3))(sin(theta)cos(theta))

help!
.

Differentiate:

. Then re-arrange to get tan 2theta = something.

There are other ways to do this, of course, but this is the second thing that popped into my head.
3. Ahh, I didn't think about changing them both in to 2theta, and that allows you to divide by cos2theta and make tan2theta.

Thankyou very much!
4. (Original post by Sir_Malc)
Ahh, I didn't think about changing them both in to 2theta, and that allows you to divide by cos2theta and make tan2theta.

Thankyou very much!
No problem.

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