Mark M. answered 03/03/22
Retired math prof. Calc 1, 2 and AP Calculus tutoring experience.
cosy + sinx = 1
-siny(dy/dx) + cosx = 0
dy/dx = cosx / siny
d2y/dx2 = [sinycosx(-sinx) - cosxcosy(dy/dx] / sin2y
= [-sinxcosxsiny - cosxcosy(cosx/siny)] / sin2y
= [-sinxcosxcosy - cos2xcoty] / sin2y