若cosα=1/7、α∈（0,π/2）求cos（α

若cosα=1/7、α∈（0,π/2）求cos（α+π/3）

2个回答

∵cosα=1/7、α∈（0,π/2）,∴sinα=(4√3)/7
cos(α+π/3)=cosα*cosπ/3-sinαsinπ/3
=1/7*1/2-(4√3)/7*√3/2=1/14-12/14=-11/14

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