函数f(x)=-X的三次方+3x的平方+9x+d
f'(x)=-3x^2+6x+9=-3(x-3)(x+1)
(-无穷,-1]或[3,+无穷）0单调递曾
f(x)在[-2,2],
f(-2)=8+12-18+d
f(2)=-8+12+18+d>f(-2)
f(2)=-8+12+18+d=2
d=-20