Question

[Nguyễn Bá Hùng]

Phân tích đa thức sau thành nhân tử:

\(ab\left(a+b\right)+bc\left(b+c\right)+ac\left(a+c\right)+2abc\)

Answer 1

[Mất nick đau lòng con quốc quốc]

\(ab\left(a+b\right)+bc\left(b+c\right)+ca\left(c+a\right)+2abc\)

\(=\)\(ab\left(a+b\right)+bc\left(a+b+c-a\right)+ca\left(c+a\right)+2abc\)

\(=\)\(ab\left(a+b\right)+bc\left(a+b\right)+bc\left(c-a\right)+ca\left(c+a\right)+2abc\)

\(=\)\(\left(a+b\right)\left(ab+bc\right)+bc\left(c-a\right)+ca\left(c+a\right)+2abc\)

\(=\)\(b\left(a+b\right)\left(c+a\right)+bc\left(c-a\right)+ca\left(c+a\right)+2abc\)

\(=\)\(b\left(ac+a^2+bc+ab\right)+b\left(c^2-ca\right)+ca\left(c+a\right)+2abc\)

\(=\)\(b\left(ca+a^2+bc+ab+c^2-ca\right)+ca\left(c+a\right)+2abc\)

\(=\)\(b\left(a^2+ab+bc+c^2\right)+ca\left(c+a\right)+2abc\)

\(=\)\(b\left(a^2+2ca+c^2+ab+bc\right)+ca\left(c+a\right)\)

\(=\)\(b\left[\left(c+a\right)^2+b\left(c+a\right)\right]+ca\left(c+a\right)\)

\(=\)\(b\left(c+a\right)\left(a+b+c\right)+ca\left(c+a\right)\)

\(=\)\(\left(c+a\right)\left(ab+b^2+bc+ca\right)\)

\(=\)\(\left(c+a\right)\left[b\left(a+b\right)+c\left(a+b\right)\right]\)

\(=\)\(\left(a+b\right)\left(b+c\right)\left(c+a\right)\)

...

Answer 2

[Nguyệt]

cách này ngắn hơn nè:

\(ab.\left(a+b\right)+bc.\left(b+c\right)+ac.\left(a+c\right)+2abc\)

\(=a^2b+ab^2+b^2c+bc^2+a^2c+ac^2+abc+abc\)

\(=\left(abc+ac^2\right)+\left(abc+b^2c\right)+\left(a^2b+ab^2\right)+\left(c^2a+c^2b\right)\)

\(=ac.\left(a+b\right)+bc.\left(a+b\right)+ab.\left(a+b\right)+c^2.\left(a+b\right)\)

\(=\left(a+b\right).\left(ac+bc+ab+c^2\right)\)

\(=\left(a+b\right).\left[c\left(a+c\right)+b.\left(a+c\right)\right]=\left(a+b\right).\left(c+b\right).\left(a+c\right)\)