M+N=\(2x^3+5x^2-4x-12+x^3+4x^2+4x+1\)
=\(3x^3+9x^2-11\)
\(2x^3+5x^2-4x-12\)
+
\(x^3+4x^2+4x+1\)
___________________
\(3x^3+9x^2-11\)
Vậy : ...
\(M+N=\left(2x^3+5x^2-4x-12\right)+\left(x^3+4x^2+4x+1\right)\)
\(M+N=2x^3+5x^2-4x-12+x^3+4x^2+4x+1\)
\(M+N=\left(2x^3+x^3\right)+\left(5x^2+4x^2\right)+\left(-4x+4x\right)+\left(-12+1\right)\)
\(M+N=3x^3+9x^2-11\)