cos a *cos 2a *cos 4a

问题描述:

cos a *cos 2a *cos 4a

==> (1/(8 * sin a) ) = (2 * sin a * cos a) * (2 * cos 2a) * (2 * cos 4a)
==> (1/(8 * sin a) ) = (sin 2a) * (2 * cos 2a) * (2 * cos 4a)
==> (1/(8 * sin a) ) = (2 * sin 2a * cos 2a) * (2 * cos 4a)
==> (1/(8 * sin a) ) = (cos 4a) * (2 * cos 4a)
==> (1/(8 * sin a) ) = (2 * sin 4a * cos 4a)
==> (1/(8 * sin a) ) = sin 8a
==> sin 8a / 8sin a