(1)y=4x^2+6x
=4(x^2+1.5x)
=4(x^2+2 X 0.75 X x + 0.75^2 - 0.75^2)
=4【(x+0.75)^2 - 0.5625】
=4(x+0.75)^2 - 2.25
它的最小值是 -2.25
(2)y=-3x^2+4x+6
=-3【x^2-(4/3)x-2】
=-3【x^2-2 X(2/3)X x + (2/3)^2 - (2/3)^2 -2】
=-3{【x-(2/3)】^2 -(22/9)}
=-3【x-(2/3)】^2 +(22/3)
最大值为22/3