问题: 二倍角
cosπ/5cos2π/5的值是多少?
解答:
cosπ/5cos2π/5
=(sinπ/5cosπ/5cos2π/5)/(sinπ/5)
=(1/2*sin2π/5cos2π/5)/(sinπ/5)
=(1/4*sin4π/5)/(sinπ/5)
=(1/4*sin(π-4π/5))/(sinπ/5)
=(1/4*sinπ/5)/(sinπ/5)
=1/4
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