\(A=\left(a+b\right)\left(b+c\right)\left(c+a\right)+abc\)
\(=a^2b+ab^2+a^2c+ac^2+b^2c+bc^2+2abc+abc\)
\(=ab\left(a+b+c\right)+bc\left(a+b+c\right)+ca\left(a+b+c\right)\)
\(=\left(a+b+c\right)\left(ab+bc+ca\right)\)
Vậy....
Đề này khó nek, ai giải hộ vs!