L{sin^3 (at)}

so for I have this:
L{ sin(at) * [1/2 ( 1 - cos(2at)]}
1/2 L{sin(at)} - 1/2 L{cos(2at)*sin(at)}
1/2 * (a^2)/(s^2 + a^2) - 1/2 L{cos(2at)*sin(at)}

How do i get rid of the cos(2at)*sin(at) ?