这样也许你能轻松明白：Y=-0.5X²-2X+7=-(0.5X²+2X-7)=-0.5(X²+4X-14)=-0.5(X²+4X+4-4-14)=-0.5(X²+4X+2²-18)=-0.5[(X+2)²-18]=-0.5(X+2)²+9