\[\boxed{\text{21\ (21).\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
Пояснение.
Решение.
\[\textbf{а)}\ (2a + 3) \cdot (2a - 3) =\]
\[= (2a)^{2} - 3^{2} = 4a^{2} - 9\]
\[\textbf{б)}\ (y - 5b) \cdot (y + 5b) =\]
\[= y^{2} - (5b)^{2} = y^{2} - \ 25b^{2}\]
\[\textbf{в)}\ (0,8x + y) \cdot (y - 0,8x) =\]
\[= y^{2} - (0,8x)^{2} = y^{2} - 0,64x^{2}\]
\[\textbf{г)}\ (b + 0,5)^{2} =\]
\[= b^{2} + 2 \cdot 0,5 \cdot b + (0,5)^{2} =\]
\[= b^{2} + b + 0,25\]
\[\textbf{д)}\ (a - 2x)^{2} =\]
\[= a^{2} - 2 \cdot 2x \cdot a + (2x)^{2} =\]
\[= a^{2} - 4ax + 4x^{2}\]
\[\textbf{е)}\ (ab - 1)^{2} =\]
\[= \left( \text{ab} \right)^{2} - 2 \cdot 1 \cdot ab + 1^{2} =\]
\[= a^{2}b^{2} - 2ab + 1\]