y'=f'(f(x))f'(x)y''=f''(f(x)))(f'(x))^2+f'(f(x))f''(x)
解：已知：y=f(f(x))有：y'=f'(f(x))·f'(x)则：y''=(y')'=[f'(f(x))·f'(x)]'=f''(f(x))·f'(x)+f'(f(fx))·f''(x)即：y''=f''(f(x))·f'(x)+f'(f(fx))·f''(x)
