Ta có
\(\left(a+b+c\right)^3=a^3+b^3+c^3+3\left(a+b\right)\left(b+c\right)\left(c+a\right)\)
\(\Rightarrow a^3+b^3+c^3=\left(a+b+c\right)^3-3\left(a+b\right)\left(b+c\right)\left(c+a\right)\)
Áp dụng vào tổng ta có
\(\left(a+b-2c\right)^3+\left(b-c-2a\right)^3+\left(c+a-2b\right)^3\)
Đặt
\(M=\left(a+b-2c+b+c-2a+a+c-2b\right)^3\)
=> M=0
\(N=3\left(a+b-2c+b+c-2a\right)\left(b+c-2a+a+c-2b\right)\left(a+c-2b+a+b-2c\right)\)
\(\Rightarrow N=3\left(c-a\right)\left(a-b\right)\left(b-c\right)\)
Để ý : M+N=B
=> \(B=0+3\left(c-a\right)\left(a-b\right)\left(b-c\right)\)
=> \(B=3\left(c-a\right)\left(a-b\right)\left(b-c\right)\)
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