a)\(\left(x^2+1\right)\left(5-x\right)\)
\(=5x^2-x^3+5-x\)
b)\(\left(x-2\right)\left(x^2+3x-4\right)\)
\(=x^3+3x^2-4x-2x^2-6x+8\)
\(=x^3+x^2-10x+8\)
c)\(\left(x-2\right)\left(x-x^2+4\right)\)
\(=x^2-x^3+4x-2x+2x^2-8\)
\(=3x^2-x^3+2x-8\)
d) Hằng đẳng thức (7)
\(\left(x-2\right)\left(x^2+2x+4\right)\)
\(=x^3-2^3=x^3-8\)