$\begin{array}{l} Áp\,dụng:{a^2} + 2ab + {b^2} = {\left( {a + b} \right)^2}\\ {a^2} - 2ab + {b^2} = {\left( {a - b} \right)^2},{a^2} - {b^2} = \left( {a - b} \right)\left( {a + b} \right)\\ 1){\left( {x - 3} \right)^2} + 2\left( {x - 3} \right)\left( {x + 5} \right) + {\left( {x + 5} \right)^2}\\ = {\left( {x - 3 + x + 5} \right)^2}\\ = {\left( {2x + 2} \right)^2} = 4{x^2} + 8x + 4\\ 2){\left( {2x - 1} \right)^2} - 2\left( {2x - 1} \right)\left( {2x + 3} \right) + {\left( {2x + 3} \right)^2}\\ = {\left( {2x + 3 - 2x + 1} \right)^2} = {4^2} = 16\\ 3){\left( {x - 3} \right)^2} - {\left( {x + 5} \right)^2}\\ = \left( {x - 3 - x - 5} \right)\left( {x - 3 + x + 5} \right)\\ = - 8\left( {2x + 2} \right)\\ = - 16x - 16 \end{array}$
