f'(x) = 3ax^2 + 2bx + cf'(x) 是一条抛物线.a > 0 则开口向上,反之,则开口向下.使导函数f'(x)>0的x的取值范围是(1,3)所以 抛物线开口向下.a < 0f'(1) = 0f'(3) = 03a + 2b + c = 027a + 6b + c = 0函数f(x)=ax^3+bx...