Đáp án:
5) 4xy
Giải thích các bước giải:
\(\begin{array}{l}
1)\left( {x + 3} \right)\left( {{x^2} + 3x - 5} \right)\\
= {x^3} + 3{x^2} - 5x + 3{x^2} + 9x - 15\\
= {x^3} + 6{x^2} + 4x - 15\\
2)2x\left( {3x - 1} \right) - \left( {2x + 1} \right)\left( {x - 3} \right) - 7x\\
= 6{x^2} - 2x - 2{x^2} + 6x - x + 3 - 7x\\
= 4{x^2} - 4x + 3\\
3)x\left( {x + y} \right) - 5x - 5y\\
= x\left( {x + y} \right) - 5\left( {x + y} \right)\\
= \left( {x + y} \right)\left( {x - 5} \right)\\
4){x^2} + 4{y^2} + 4xy\\
= {x^2} + 2.x.2y + {\left( {2y} \right)^2}\\
= {\left( {x + 2y} \right)^2}\\
5){\left( {x + y} \right)^2} - {\left( {x - y} \right)^2}\\
= \left( {x + y - x + y} \right)\left( {x + y + x - y} \right)\\
= 2y.2x = 4xy
\end{array}\)
