求值cos5π/8cosπ/8
问题描述：
求值cos5π/8cosπ/8
解cos5π/8cosπ/8,请写出过程.
答
cos(5π/8+π/8)=cos5π/8cosπ/8-sin5π/8sinπ/8=-√2/2
cos(5π/8-π/8)=cos5π/8cosπ/8+sin5π/8sinπ/8=0
两式相加2cos5π/8cosπ/8=-√2/2
cos5π/8cosπ/8=-√2/4