已知函数f(x)=ax^3+bx^2+cx+d的两个极值点是-1和3,且f(0)=-7,f`(0)=-18,求f(x)的表达式
问题描述：
已知函数f(x)=ax^3+bx^2+cx+d的两个极值点是-1和3,且f(0)=-7,f`(0)=-18,求f(x)的表达式
答
f(0) = d = -7
f'(x) = 3ax² + 2bx + c
f'(0) = c = -18
f'(-1) = 3a - 2b - 18 = 0
f'(3) = 27a + 6b -18 = 0
a = 2,b = -6
y = 2x³ - 6x² - 18x - 7