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x?+2xy=-2,4x?+8xy=-8①xy-y?=-4②4x?功行川跳还支久+7xy+y?①-②:(4x?+8xy)-(xy-y?)=-8-(-4)4x?+8xy-xy+y?=-8+44x?+7船调玉征危附陆容联叫度xy+y?=-4
用代数式求值形式4x?+7xy+y?=4x?+(8-1)xy+y?=4x?+8xy-xy+y?=争肉游(4x?+8xy)+(-xy+y?)=4(x?+2xy)-(xy-y?)【代入】=4*(-2)-(-4)=-8+4=-4用解方程组形式x?+2xy=-2①x随制高散架个攻y-y?=-4②①问答×4-②4(x?+xy)-(xy-y?)=4(-2)-(-4)即4x?+7xy+y?=-4