问题描述: 已知sinα=2/3,α∈(π/2,π),求cos(π/3+α),cos(π/3-α) 1个回答 分类:数学 2014-10-27 问题解答: 我来补答 sinα=2/3,α∈(π/2,π),∴cosα=-(1-sin²α)½=-√5/3∴cos(π/3+α)=cosπ/3·cosα-sinπ/3·sinα=1/2·(-√5/3)-√3/2·2/3=-(√5+2√3)/6cos(π/3-α)=cosπ/3·cosα+sinπ/3·sinα=1/2·(-√5/3)+√3/2·2/3=(2√3-√5)/6 展开全文阅读