P/s : Phần b ) : \(\left(x+a\right)\left(x+b\right)\left(x+c\right)\)
a ) \(\left(x+a\right)\left(x+b\right)=x^2+ax+bx+ab=x^2+\left(a+b\right)x+ab\)
b ) \(\left(x+a\right)\left(x+b\right)\left(x+c\right)\)
\(=\left[x^2+\left(a+b\right)x+ab\right]\left(x+c\right)\)
\(=x^2\left(x+c\right)+\left(a+b\right)x\left(x+c\right)+ab\left(x+c\right)\)
\(=x^3+x^2c+\left(ax+bx\right)\left(x+c\right)+abx+abc\)
\(=x^3+x^2c+ax^2+bx^2+axc+bxc+abx+abc\)
\(=x^3+\left(x^2a+x^2b+x^2c\right)+\left(abx+bcx+axc\right)+abc\)
\(=x^3+\left(a+b+c\right)x^2+\left(ab+bc+ca\right)x+abc\)