F(x)=lim(t->∞) t^2*[f(x+π/t)-f(x)]*sin(x/t)=lim(t->∞) t^2*(x/t)*(π/t)*[f(x+π/t)-f(x)]/(π/t)=πx*lim(t->∞) [f(x+π/t)-f(x)]/(π/t)=πx*f'(x)所以F'(x)=πf'(x)+πx*f''(x)
