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Deepak Bora
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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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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

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  1. 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]

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