>>614
f(x)＝ｘ^2cos3x

{f(x+h)-f(x)}/h
= {(x+h)^2*cos(3x+3h) - x^2*cos3x}/h
= x^2*(cos(3x+3h)-cos3x)/h + 2x*cos(3x+3h) + h*(cos3x+3h)
= x^2*{-2sin3x*sin(3h/2)}/h + 2x*cos(3x+3h) + h*(cos3x+3h)
= -3*x^2*sin3x*sin(3h/2)/(3h/2) + 2x*cos(3x+3h) + h*(cos3x+3h)　　(1)
→ -3*x^2*sin3x + 2x*cos3x


(1)
cos(x+y)=cosx*cosy-sinx*siny
cos(x-y)=cosx*cosy+sinx*siny