已知x2-xy=-3,2xy-y2=-8,求 (1)x2+xy-y2 (2)2x2+4xy-3y2 字母后面直接跟数字的是平方

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1.x²+xy-y²
=x²-xy+2xy-y²
=-3+(-8)
=-11
2.2x²+4xy-3y²
=2(x²-xy)+3(2xy-y²)
=-6-24
=-30

x2+xy-y2=x2-xy+2xy-y2=-11
2x2+4xy-3y2=2(x2-xy)+3(2xy-y2)=2*(-3)+3*(-8)=-30

x2-xy=-3 (1)
2xy-y2=-8 (2)
(1)+(2),得
x2+xy-y2 = -3-8
=-11
∴(1) x2+xy-y2 = -11
(2)2x2+4xy-3y2
=2(x2+xy-y2 )+2xy-y2
把x2+xy-y2 = -11和2xy-y2=-8 代人上式，得
2x2+4xy-3y2
=2(x2+xy-y2 )+2xy-y2
=2×(-11)+(-8)
= -22-8
= -30

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