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# C4 differentiation watch

1. Ok two things.

1)

Implicitly differentiate cos(x+y) = siny

Have i done it correct?:
-sin(x+y).(1+ dy/dx) = cosy.(dy/dx)
-dy/dx.sin(x+y) - sin(x+y) = cosy.(dy/dx)
-sin(x+y) = cosy.(dy/dx) + (dy/dx).sin(x+y)

dy/dx = -sin(x+y) / cosy + sin(x+y)

2) curve C has parametric equations x=t3 and y=t2. Find an equation for the tangent to C at A(1,1).

I get y = (2/3)x + (1/3)

However this is not the answer in the book... What am i doing wrong?

Thanks,

Wiz
2. I got both answers the same as yours.
3. cool, the books wrong then?
5. the book doesnt have an answer for the first Q, thats why i was asking if it was right, and for the second one the book says the answer is y = (1/3)x + (1/3)
6. (Original post by __WiZaRD__)
the book doesnt have an answer for the first Q, thats why i was asking if it was right, and for the second one the book says the answer is y = (1/3)x + (1/3)
well then ya... the book has some mistakes...

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Updated: June 13, 2006
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