alleryIn triangle with sides 7, 8, 9, find cos of the angle opposite to side 9 (use cosine rule):
A11/16
B1/2
C2/7
D1/7
Answer & Solution
Correct answer: C. 2/7
**Cosine rule:** c² = a² + b² - 2ab cosC.
Here c = 9, a = 7, b = 8. 81 = 49 + 64 - 2(7)(8)cosC = 113 - 112 cosC.
**So 112 cosC = 113 - 81 = 32, cosC = 32/112 = 2/7.**
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