y=-0.5(x-2)²+5≤5即y的最大值为5求计算过程。。。y=3+2x-0.5x²=-0.5(x²-4x)+3=-0.5(x²-4x+4-4)+3=-0.5(x²-4x+4)-0.5×(-4)+3=-0.5(x-2)²+5