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# Differentiation Core 3 Watch

1. How would you find dx/dy of x = cos(2y + pi ) ??

Can you not do..

x = cos2y + cospi
then dx/dy = -2sin2y - sin pi?
2. (Original post by stanmoor)
How would you find dx/dy of x = cos(2y + pi ) ??

Can you not do..

x = cos2y + cospi
then dx/dy = -2sin2y - sin pi?
You have to use the addition formula. (cos2y x cospi) - (sin2y x sinpi)

Not sure how to differentiate it since thats not in our C3.
3. correct me if im wrong, but would you not use chain rule: letting u=2y+pi then x=cosu
then find dx/dy
and finally find the reciprocal to find dy/dx

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