Geometryhard · Past Paper
In triangle ABC, side b = 20, c = 21 and sin A = 3/5. Find side a.
A13
B29
C17
D25
✓ Correct Answer: A — 13
cos A = 4/5 (assuming A is acute). a^2 = b^2 + c^2 - 2bc cos A = 400 + 441 - 2(20)(21)(4/5) = 841 - 672 = 169. a = 13.
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