(cosπ/5)(cos2π/5)的值等于
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(cosπ/5)(cos2π/5)的值等于
(cosπ/5)(cos2π/5)的值等于
(cosπ/5)(cos2π/5)的值等于
先把它看成分母为1的分数,(cosπ/5)(cos2π/5)/1,然后分子分母同时乘sin(π/5),这样分子上可以用一下sin的二倍角公式式子变为:sin(2π/5)*cos(2π/5)/2sin(π/5),分子上再用一下sin的二倍角公式,就能得到:sin(4π/5)/4sin(π/5),由诱导公式可以知道sin(4π/5)=sin(π/5).所以原式等于1/4.
(cosπ/5)(cos2π/5)的值等于
cosπ/5*cos2π/5的值等于
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