Ответ:
\[25a^{2} - 10ab + \ *\ \ \]
\[25a^{2} - 2 \cdot 5ab + b^{2}\]
\[*\ = b^{2}\]
\[25a^{2} - 10ab + b^{2} = (5a - b)^{2}\text{.\ }\]
\[25a^{2} - 10ab + \ *\ \ \]
\[25a^{2} - 2 \cdot 5ab + b^{2}\]
\[*\ = b^{2}\]
\[25a^{2} - 10ab + b^{2} = (5a - b)^{2}\text{.\ }\]