1.

B+C4.In a triangleABC, write cos|一2의 in terms of angle A.

Answer»

A+B+C = 180 (ASP of trianlge)

~ B + C = 180 - A

~ cos (B + C) / 2 = cos (180 - A) / 2

~ cos (180/2 - A/2)

~ cos (90 - A/2)

~ sin (A/2)

Therefore, cos (B + C/2) = sin (A/2)

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