find d^2y/dx^2 in terms of x and y

Question

sin²(y) + cos²(x) = 1

Answer 1

if sin²y + cos²x = 1 then sin²y = 1 - cos²x = sin²x

But, if

sin²y = sin²x then y = x !

Then,

dy/dx = 1 and d²y/dx² = 0