\[\boxed{\text{990.}\text{\ }еуроки - ответы\ на\ пятёрку}\]
\[\textbf{а)}\ (x - 8)(x + 8) -\]
\[- (x - 12)(x + 12) = x^{2} -\]
\[- 64 - x^{2} + 144 = 80\]
\[\textbf{б)}\ \left( y - \frac{5}{9} \right)\left( y + \frac{5}{9} \right) +\]
\[+ \left( \frac{2}{3} - y \right)\left( \frac{2}{3} + y \right) = y^{2} -\]
\[- \frac{25}{81} + \frac{4}{9} - y^{2} =\]
\[= \frac{- 25 + 4 \cdot 9}{81} =\]
\[= \frac{- 25 + 36}{81} = \frac{11}{81}\]