cos1/5π*cos2/5π

问题描述:

cos1/5π*cos2/5π
这个式子该怎么下手?

cos1/5π*cos2/5π=(2sin1/5π*cos1/5π*cos2/5π)/(2sin1/5π)=(sin2/5π*cos2/5π)/(2sin1/5π)=(sin4/5π)/(4sin1/5π)=(sin1/5π)/4sin1/5π=1/4