Thực hiện phép tính:
a. \(\left( {x - 3} \right)\left( {{x^2} + 4x + 5} \right)\)
b. \(\left( {2{x^3} - 4{x^2} - 5x + 7} \right):\left( {x - 1} \right)\)
Giải chi tiết:
\(\begin{array}{l}a)\,\,\left( {x - 3} \right)\left( {{x^2} + 4x + 5} \right) = {x^3} + 4{x^2} + 5x - 3{x^2} - 12x - 15 = {x^3} + {x^2} - 7x - 15\\b)\;\left( {2{x^3} - 4{x^2} - 5x + 7} \right):\left( {x - 1} \right)\\ = \left( {2{x^3} - 2{x^2} - 2{x^2} + 2x - 7x + 7} \right):\left( {x - 1} \right)\\ = \left[ {2{x^2}\left( {x - 1} \right) - 2x\left( {x - 1} \right) - 7\left( {x - 1} \right)} \right]:\left( {x - 1} \right)\\ = \left( {x - 1} \right)\left( {2{x^2} - 2x - 7} \right):\left( {x - 1} \right)\\ = 2{x^2} - 2x - 7.\end{array}\)
Chọn A.