A . Maths

a) L.H.S. = ( sin 5x / sin x ) - ( cos 5x / cos x )
= ( sin 5x cos x - sin x cos 5x ) / sin x cos x
= [ sin 6x + sin 4x - sin 6x - sin ( -4x ) ] / 2 sin x cos x
= 2 sin 4x / 2 sin x cos x
= 4 sin 2x cos 2x / sin 2x
= 4 cos 2x
= R.H.S.
b) 用a的結果:
4 cos 2x = 2
cos 2x = 1 / 2
2x = 360*n + / - 60*
x = 180*n + / - 30* ( n是整數 )

2008-07-05 19:14:03 補充：
所用公式: 2 sin A cos B = sin ( A + B ) + sin ( A - B )
sin 2x = 2 sin x cos x