为什么cos5π/12等于sinπ/12

为什么cos5π/12等于sinπ/12 cos5π/12=sinπ/12 不查表求值：cos5π/12·sinπ/12= (sin5π/12+cos5π/12)(sinπ/12-cosπ/12) (sin5π/12-sinπ/12)(cos5π/12+cosπ/12)=? cos5/12πcosπ/12+cosπ/12sinπ/6=? sin5/12π×cos5/12π怎么算? 计算cos5分之π+cos5分之2π+cos5分之3π+cos5分之4π cos方5π/12+cos方π/12+cosπ/12*cos5π/12 最后等于5/4的过程. cos方5π/12+cos方π/12+cosπ/12*cos5π/12 最后等于4/5的过程. 例题9和10,我知道有解析,但就是看不懂,好像是公式吧,我想知道cos5π/12是怎么变成sin的,周期不是π/2吗,乘 数学极限的运算lim α→π/2 （sinα-sin5α）/（cosα+cos5α）