已知A=x²-xy+y²,B=-x²+2xy+y².求-A-2B的值,其中x=3,y=-1/3
问题描述：
已知A=x²-xy+y²,B=-x²+2xy+y².求-A-2B的值,其中x=3,y=-1/3
答
-A-2B
=-(x²-xy+y²)-2(-x²+2xy+y²)
=-x²+xy-y²+2x²-4xy-2y²
=x²-3xy-3y²
=3²-3*3*(-1/3)-3*(-1/3)²
=9+3-1/3
=12-1/3
=(36-1)/3
=35/3
答
87/9
答
-A-2B=-x^2+xy-y^2+2x^2-4xy-2y^2=x^2-3xy-3y^2,将x=3,y=-1/3代入上式,得3^2+3*3*1/3-3*(-1/3)^2=9+3-1/3=12-1/3=35/3