y=2(x^2-3x)+5 =2(x^2-3x+9/4-9/4)+5 =2(x-3/2)^2-2*9/4+5 =2(x-3/2)^2+1/2所以此函数有最小值 当x=3/2时 y最大值为1/2y=-2(x^2+3x)+5 =-2(x^2+3x+9/4-9/4)+5 =-2(x+3/2)^2+2*9/4+5 =-2(x+3/2)^2+19/2所以此函数有最大值 当x=-3/2时 y最大值为19/2
