cos1/5π*cos2/5π

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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

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