Раскройте скобки и упростите: 5a-(6a-(7a-(8a-9))).

\[5a - \left( 6a - \left( 7a - (8a - 9) \right) \right) =\]

\[= 5a - 6a + \left( 7a - (8a - 9) \right) =\]

\[= - a + 7a - (8a - 9) =\]

\[= 6a - 8a + 9 = - 2a + 9.\]