Đáp án:
$\begin{array}{l}
4)a){\left( {a + b} \right)^2} - {\left( {a - b} \right)^2}\\
= \left( {a + b + a - b} \right)\left( {a + b - a + b} \right)\\
= 2a.2b\\
= 4ab\\
b)\\
2\left( {x - y} \right)\left( {x + y} \right) + {\left( {x + y} \right)^2} + {\left( {x - y} \right)^2}\\
= {\left( {x + y} \right)^2} + 2\left( {x + y} \right)\left( {x - y} \right) + {\left( {x - y} \right)^2}\\
= {\left( {x + y + x - y} \right)^2}\\
= {\left( {2x} \right)^2}\\
= 4{x^2}\\
5)a)\\
{34^2} + {66^2} + 68.66\\
= {34^2} + 2.34.66 + {66^2}\\
= {\left( {34 + 66} \right)^2}\\
= {100^2}\\
= 10000\\
b){74^2} + {24^2} - 48.74\\
= {74^2} - 2.24.74 + {24^2}\\
= {\left( {74 - 24} \right)^2}\\
= {50^2}\\
= 2500\\
6)\\
a)\left( {3x + y} \right)\left( {9{x^2} - 3xy + {y^2}} \right) = 27{x^3} + {y^3}\\
b)\left( {2x - 5} \right)\left( {4{x^2} + 10x + 25} \right)
\end{array}$
