2X²-3X+1+(-3X²+5X-7)
=(2-3)x^2+(-3+5)x+(1-7)
=-x^2+2x-6
-X²+3XY-0.5y²-(-0.25X²+4XY-二分之三Y^2)
=(-1+0.25)x^2+(3+4)xy+(-0.5+3/2)y^2
=-0.75x^2+7xy+y^2