若3x^2-4xy+y^2=-3,2x^2+3xy-2y^2=8,求5x^2-18xy+7y^2的值求解题过程,今晚就要!

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若3x^2-4xy+y^2=-3,2x^2+3xy-2y^2=8,求5x^2-18xy+7y^2的值求解题过程,今晚就要!
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若3x^2-4xy+y^2=-3,2x^2+3xy-2y^2=8,求5x^2-18xy+7y^2的值求解题过程,今晚就要!
若3x^2-4xy+y^2=-3,2x^2+3xy-2y^2=8,求5x^2-18xy+7y^2的值求解题过程,今晚就要!

若3x^2-4xy+y^2=-3,2x^2+3xy-2y^2=8,求5x^2-18xy+7y^2的值求解题过程,今晚就要!
3x^2-4xy+y^2=-3, (1) 2x^2+3xy-2y^2=8,(2),
(1)X3-(2)X2得：5x^2-18xy+7y^2=-3x3-8x2=-25


3x²-4xy+y²=-3 标为 ①
2x²+3xy-2y²=8 标为 ②
3×①-2×②
3（3x²-4xy+y²）-2（2x²+3xy-2y²）=3×（-3）-2×8
9x²-12xy+3y²-4x²-6xy+4y²=-9-16
9x²-4x²-12xy-6xy+3y²+4y²=-25
5x²-18xy+7y²=-25

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