Упростите выражение: (a-c)(a+c)-(a-2c)^2.

\[(a - c)(a + c) - (a - 2c)^{2} =\]

\[= a^{2} - c^{2} - \left( a^{2} - 4ac + 4c^{2} \right) =\]

\[= a^{2} - c^{2} - a^{2} + 4ac - 4c^{2} =\]

\[= - 5c^{2} + 4ac.\]