Question

试说明代数式-2x平方+4x-18有最小值,并求出这个最大值,此时x为多少?

Answer 1

设y=-2x^2+4x-18,

∵a=-2＜0,

故y有最大值,即

(4ac-b^2)/4a

=[4×(-2)×(-18)-4^2] / 4×(-2)

=-16

x=-b/2a=-4/2*(-2)=1