Use implicit differentiation to find the second derivative:
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Up to this place all's right, but now you have to differentiate the right side, and get . I can't see from where that 3 in the power of the denominator in what you call "the answer" comes...
My textbook has answers to all the odd numbered questions. The denominator is definitely cubed.
& with the differentiation, I'm still getting -sin y not sin y.
It is a quotient derivative: , and since the derivative of cos x is -sin x , with the sign minus it must be plus...and squared in the denominator.
What book do you have?