先化简,再求值:5x²-2(3y²+7xy)+(2y²-5x²),其中x=1,y=-2
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先化简,再求值:5x²-2(3y²+7xy)+(2y²-5x²),其中x=1,y=-2
先化简,再求值:5x²-2(3y²+7xy)+(2y²-5x²),其中x=1,y=-2
先化简,再求值:5x²-2(3y²+7xy)+(2y²-5x²),其中x=1,y=-2
5x²-2(3y²+7xy)+(2y²-5x²)
=5x²-5x²- 2(3y²+7xy)+2y²
= -2(3y²-y²+7xy)
=-2(2y²+7xy)
=-2*(2*(-2)²+7*1*-2)
=-2*(2*4-14)
=-2*-6
=12
口算得-14xy-4y2=-2y(7x+2y)
带入数据得12