求值cos5π/8*cosπ8

问题描述:

求值cos5π/8*cosπ8

cos5π/8*cosπ/8
=-cos(π-5π/8)*cosπ/8
=-cos3π/8*cosπ/8
=-cos(π/2-π/8)*cosπ/8
=-sin(π/8)*cosπ/8
=-sin(π/4)/2
=-√2/4