\(x^3+y^3=\left(x+y\right)\left(x^2-xy+y^2\right)\)
Chứng minh: \(VP=\left(x+y\right)\left(x^2-xy+y^2\right)=x^3-x^2y+xy^2+x^2y-xy^2+y^3=x^3+y^3=VP\)
Áp dụng vào bài
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Ta có \(a+b+c=0\Leftrightarrow-c=a+b\)
\(\Rightarrow c^2=\left(a+b\right)\left(a+b\right)=a^2+2ab+b^2\)
Xét \(a^3+b^3+a^2c+b^2c-abc\)
\(=a^3+b^3+c\left(a^2+b^2+2ab\right)-3abc\)
\(=a^3+b^3+c.c^2-3abc\)
\(=a^3+b^3+c^3-3abc\)
\(=a^3+a^2b+2a^2b+2ab^2+ab^2+b^3-3a^2b-3ab^2+c^3-3abc\)
\(=a^2\left(a+b\right)+2ab\left(a+b\right)+b^2\left(a+b\right)+c^3-3ab\left(a+b+c\right)\)
\(=\left(a+b\right)\left(a^2+2ab+b^2\right)+c^3\) ( do a+b+c=0 )
\(=\left(a+b\right)\left[a\left(a+b\right)+b\left(a+b\right)\right]+c^3\)
\(=\left(a+b\right)\left(a+b\right)\left(a+b\right)+c^3=\left(a+b\right)^3+c^3\)
( Áp dụng \(x^3+y^3=\left(x+y\right)\left(x^2-xy+y^2\right)\) )
\(=\left(a+b+c\right)\left[\left(a+b\right)^2-\left(a+b\right)c+c^2\right]=0\) ( do a+b+c=0 )
Vậy \(a^3+b^3+a^2c+b^2c-abc=0\)
