Show that Ө = β /3
( 2cos2Ө - 2cos(β - Ө))
更新1:
cos2Ө = cos(β - Ө) Why do you cancel the cos in both side?
更新2:
0< Ө < 90 < β < 180 ????
更新3:
If Ө = 70, β = 210 ??? Out of the range.
