Thực hiện phép tính: a) ( x -3 ) . ( x^ 2 - x + 5 )

a) \(\left( {x - 3} \right).\left( {{x^2} - x + 5} \right)\)

\( = {x^3} - {x^2} + 5x - 3{x^2} + 3x - 15\)

\( = {x^3} - 4{x^2} + 8x - 15\).

b)\[\frac{{{x^2}}}{{3x + 6}} + \frac{{4x + 4}}{{3x + 6}}\]

\( = \frac{{{x^2} + 4x + 4}}{{3x + 6}}\)

\( = \frac{{{{\left( {x + 2} \right)}^2}}}{{3.\left( {x + 2} \right)}} = \frac{{x + 2}}{3}\).