* làm gọn hơn chút:
\(VT=\left(a+b\right)\left(a^2-ab+b^2\right)+\left(c+d\right)\left(c^2-cd+d^2\right)\)
\(=-\left(c+d\right)\left(a^2-ab+b^2\right)+\left(c+d\right)\left(c^2-cd+d^2\right)\)
\(=\left(c+d\right)\left(c^2-cd+d^2-a^2+ab-b^2\right)\)
\(=\left(c+d\right)\left[\left(c^2+d^2\right)+\left(ab-cd\right)-\left(a^2+b^2\right)\right]\)
\(=\left(c+d\right)\left[\left(c+d\right)^2-2cd+\left(ab-cd\right)-\left(a+b\right)^2+2ab\right]\)
\(=\left(c+d\right)\left[\left(c+d+a+b\right)\left(c+d-a-b\right)+3\left(ab-cd\right)\right]\)
\(=\left(c+d\right).3\left(ab-cd\right)\left(đpcm\right)\)
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giúp mình bài này với ạ,ghi từng bước giúp mình lun ạ
