Question

if a b and c are interior angles of a triangle abc then cos(b+c)/2

Answer 1

Step-by-step explanation:

Sin[(B+C)/2]

Since A+B+C=180 for interior angles of triangle ABC.

then B+C=180-A.

NOW   Sin [(180-A)/2]

             =Sin[90-(A/2)]                      since Sin(90-A)=CosA

              =Cos(A/2)

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