1.若2x+7/ax+1是一个多项式,则a= 2.已知a+b=3,ab=1,求1/2(a-b)^2的值. 3.已知(1+M)(1+3a+N)=1-27a^3,M= ,N4.已知(x+2y)^2=24,(x-2y)^2=20,则xy=____.(要过程)
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![1.若2x+7/ax+1是一个多项式,则a= 2.已知a+b=3,ab=1,求1/2(a-b)^2的值. 3.已知(1+M)(1+3a+N)=1-27a^3,M= ,N4.已知(x+2y)^2=24,(x-2y)^2=20,则xy=____.(要过程)](/uploads/image/z/11556379-19-9.jpg?t=1.%E8%8B%A52x%2B7%2Fax%2B1%E6%98%AF%E4%B8%80%E4%B8%AA%E5%A4%9A%E9%A1%B9%E5%BC%8F%2C%E5%88%99a%3D+2.%E5%B7%B2%E7%9F%A5a%2Bb%3D3%2Cab%3D1%2C%E6%B1%821%2F2%28a-b%29%5E2%E7%9A%84%E5%80%BC.+3.%E5%B7%B2%E7%9F%A5%281%2BM%29%281%2B3a%2BN%29%3D1-27a%5E3%2CM%3D+%2CN4.%E5%B7%B2%E7%9F%A5%28x%2B2y%29%5E2%3D24%EF%BC%8C%EF%BC%88x-2y%29%5E2%3D20%2C%E5%88%99xy%3D____.%28%E8%A6%81%E8%BF%87%E7%A8%8B%EF%BC%89)
1.若2x+7/ax+1是一个多项式,则a= 2.已知a+b=3,ab=1,求1/2(a-b)^2的值. 3.已知(1+M)(1+3a+N)=1-27a^3,M= ,N4.已知(x+2y)^2=24,(x-2y)^2=20,则xy=____.(要过程)
1.若2x+7/ax+1是一个多项式,则a= 2.已知a+b=3,ab=1,求1/2(a-b)^2的值. 3.已知(1+M)(1+3a+N)=1-27a^3,M= ,N
4.已知(x+2y)^2=24,(x-2y)^2=20,则xy=____.(要过程)
1.若2x+7/ax+1是一个多项式,则a= 2.已知a+b=3,ab=1,求1/2(a-b)^2的值. 3.已知(1+M)(1+3a+N)=1-27a^3,M= ,N4.已知(x+2y)^2=24,(x-2y)^2=20,则xy=____.(要过程)
1.a=0
2.1/10
3.M=-3a,N=9a^2
4.1/2
将a+b=3,ab=1代入1/2(a-b)^2=1/2(a+b)^2-2ab=1/2*3^2-2*1=2.5
(1+M)(1+3a+N)=1-27a^3,
展开(1+M)(1+3a+N)=3a((1+M)+(1+M)*(1+N)
观察3a((1+M)+(1+M)*(1+N)=1-27a^3可知1+M=-9a^2,(1+M)*(1+N)=1
由1+M=-9a^2解得M...
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将a+b=3,ab=1代入1/2(a-b)^2=1/2(a+b)^2-2ab=1/2*3^2-2*1=2.5
(1+M)(1+3a+N)=1-27a^3,
展开(1+M)(1+3a+N)=3a((1+M)+(1+M)*(1+N)
观察3a((1+M)+(1+M)*(1+N)=1-27a^3可知1+M=-9a^2,(1+M)*(1+N)=1
由1+M=-9a^2解得M=-9a^2-1
将1+M=-9a^2代入,(1+M)*(1+N)=1得N=-1/(9a^2)-1
已知(x+2y)^2=24,(x-2y)^2=20
所以有已知(x+2y)^2-(x-2y)^2=24-20=4
因为(x+2y)^2-(x-2y)^2=8xy
所以8xy=4
xy=0.5
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