当x=2,y=2/5时,(x-y)(x-2y)-1/2(2x-3y)(x+2y)的值为( )四个选项:a.-2/3 b.2/3 c.-2 d.2我的财富值不多了,望请见谅,
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![当x=2,y=2/5时,(x-y)(x-2y)-1/2(2x-3y)(x+2y)的值为( )四个选项:a.-2/3 b.2/3 c.-2 d.2我的财富值不多了,望请见谅,](/uploads/image/z/3010742-62-2.jpg?t=%E5%BD%93x%3D2%2Cy%3D2%2F5%E6%97%B6%2C%28x-y%29%28x-2y%29-1%2F2%282x-3y%29%28x%2B2y%29%E7%9A%84%E5%80%BC%E4%B8%BA%EF%BC%88+%EF%BC%89%E5%9B%9B%E4%B8%AA%E9%80%89%E9%A1%B9%EF%BC%9Aa.-2%2F3+b.2%2F3+c.-2+d.2%E6%88%91%E7%9A%84%E8%B4%A2%E5%AF%8C%E5%80%BC%E4%B8%8D%E5%A4%9A%E4%BA%86%2C%E6%9C%9B%E8%AF%B7%E8%A7%81%E8%B0%85%2C)
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当x=2,y=2/5时,(x-y)(x-2y)-1/2(2x-3y)(x+2y)的值为( )四个选项:a.-2/3 b.2/3 c.-2 d.2我的财富值不多了,望请见谅,
当x=2,y=2/5时,(x-y)(x-2y)-1/2(2x-3y)(x+2y)的值为( )
四个选项:a.-2/3 b.2/3 c.-2 d.2
我的财富值不多了,望请见谅,
当x=2,y=2/5时,(x-y)(x-2y)-1/2(2x-3y)(x+2y)的值为( )四个选项:a.-2/3 b.2/3 c.-2 d.2我的财富值不多了,望请见谅,
原式=x²-3xy+2y²-x²-1/2xy+3y²
=-7/2xy+5y²
=-14/5+4/5
=-2
选c
x=2,y=2/5
(x-y)(x-2y)-1/2(2x-3y)(x+2y)
=(x-y)(x-2y)-1/2(2x-2y-y)(x+2y)
=(x-y)(x-2y)-(x-y)(x+2y) +1/2 y(x+2y)
=(x-y)(x-2y-x-2y) +1/2 y(x+2y)
=-4y(x-y) +1/2 y(x+2y)
=-4xy + 4y^2 +1/2xy+y^2
=-7/2xy+5y^2
=-7/2*2*2/5 + 5*(2/5)^2
=-14/5+4/5
=-2