若X-Y=3则X²-3Y-XY的值为

若X-Y=3则X²-3Y-XY的值为
若X-Y=3则X²-3Y-XY的值为

若X-Y=3则X²-3Y-XY的值为
x²-3y-xy
=(x²-xy)-3y
=x(x-y)-3y
=3x-3y
=3(x-y)
=3×3
=9

x^2-3y-xy=x^2-(x-y)y-xy=x^2-2xy+y^2=(x-y)^2=9

原式:X-Y=3则X=3+Y=Y+3
X^2-3Y-XY=(Y+3)^2-3Y-(Y+3)Y=Y^2+6Y+9-3Y-Y^2-3Y=9