CBSE 10th Class Mathematics
Past year question paper
Year 2020
SET 1, Code Number – 30/5/1
Question Number – 23
Trigonometric Function
Use trigonometric relation
Deepak BoraNewbie
If A, B and C are interior angles of a ΔABC, then show that cos[ (B + C) / 2 ] = sin (A/2)
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Given A, B and C are interior angles of ΔABC,
A + B + C = 180° (Angle sum property of triangle)
B + C = 180 – A
∴ [B + C] / 2 = [180-A] /2 = 90 – [A/2]
∴ COS [B + C] / 2 = COS 90 – [A/2]
∴ COS [B + C] / 2 = SIN [A/2]