\[5 \cdot (a + b)^{2} - 10ab = 5 \cdot \left( a^{2} + 2ab + b^{2} \right) -\]
\[- \ 10ab = 5a^{2} + 10ab + 5b^{2} - 10ab =\]
\[= 5a^{2} + 5b^{2}\]