What is its worth 15 ∘ ?

Question

What is its worth       15          ∘      ?

Monserrat Wong · Accepted Answer

Find the value of    cos  ⁡      15          ∘      We know that,  cos  =  cos  ⁡  (      45          ∘        −      30          ∘        )  (      15          ∘        )  =  cos  ⁡      45          ∘        cos  ⁡      30          ∘        +  sin  ⁡      45          ∘        sin  ⁡      30          ∘          {  ∵  cos  ⁡  (  A  −  B  )  =  cos  ⁡  A  cos  ⁡  B  +  sin  ⁡  A  sin  ⁡  B  }  =  (      1          2        )  (            3        2    )  +  (      1          2        )  (      1    2    )    {  P  u  t  cos  ⁡      45          ∘        =      1          2        ,  cos  ⁡      30          ∘        =            3        2    ,  sin  ⁡      45          ∘        =      1          2        sin  ⁡      30          ∘        =      1    2    }  =            3              2              2              +      1          2              2              =                    3            +      1              2              2            Thus, the required value is   cos  ⁡  (      15          ∘        )  =                    3            +      1              2              2

What is the value of cos 15^{circ}?

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