Differentiating with parametric equations

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March 18th 2012, 06:52 AM
johnreal

Differentiating with parametric equations

For part of my parametric eqn i need to differentiate 3-2cos2 theta, when i do this i get 2-sin2 theta, but the answers says 4sin2 theta, what am i doing wrong(Wondering)
March 18th 2012, 06:58 AM
princeps

Re: Differentiating with parametric equations

Quote:

Originally Posted by johnreal

For part of my parametric eqn i need to differentiate 3-2cos2 theta, when i do this i get 2-sin2 theta, but the answers says 4sin2 theta, what am i doing wrong(Wondering)

$(3-2\cos \2\theta)'=3'-(2\cdot (\cos \2\theta)'+2' \cdot \cos \2 \theta)=$

$= 0-(-4\cdot \sin \2 \theta +0)= 4 \cdot \sin 2 \theta$
March 18th 2012, 07:18 AM
johnreal

Re: Differentiating with parametric equations

Thanks alot this helped *slaps face* Product Rule, ive worked out most of it and ive reached sin2 theta/cos theta, i did that by having 4sin2 theta/4cos theta and cancelling the fours, im supposed to get 2 sin theta, did i do some thing wrong?
March 18th 2012, 07:27 AM
princeps

Re: Differentiating with parametric equations

Quote:

Originally Posted by johnreal

Thanks alot this helped *slaps face* Product Rule, ive worked out most of it and ive reached sin2 theta/cos theta, i did that by having 4sin2 theta/4cos theta and cancelling the fours, im supposed to get 2 sin theta, did i do some thing wrong?

$\sin 2\theta = 2 \sin \theta \cdot \cos \theta$

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