(√10 t )' = ((10 t )^(1/2))' = (1/2) ((10 t )^(-1/2)) (10 t )' = 5 (10 t )^(-1/2) 先求f'(x) = sinx + x cosx - sinx = x cosx 再求 f'(π/2) = x cosx = π/2 (0) = 0