已知A=-x^+2xy-3y^,B=5x^-xy+2y^ 求:(1)4A-6B; (2)若2A+B+C=0,求C.
问题描述:
已知A=-x^+2xy-3y^,B=5x^-xy+2y^ 求:(1)4A-6B; (2)若2A+B+C=0,求C.
答
-3x²-3xy+4y²
答
因为A=-x²+2xy-3y²,B=5x²-xy+2y²
所以(1)4A-6B
=4*(-x²+2xy-3y²)-6(5x²-xy+2y²)
=-4x²+8xy-12y²-30x²+6xy-12y²
=-34x²+14xy-24y²
(2) 若2A+B+C=0
则C=-(2A+B)
=-[2(-x²+2xy-3y²)+(5x²-xy+2y²)]
=-(3x²+3xy-4y²)
=-3x²-3xy+4y²