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Question
In ΔABC, a(cos2B + cos2C) + cos A(c cos C + b cos B) = ?
Options
0
a
b
c
MCQ
Solution
a
Explanation:
a(cos2 B + cos2 C) + cosA (c cos C + b cos B)
= a cos2 B + a cos2 C + c cos A cos C+ b cos A cos B
= cos B(a cos B + b cos A) + cos C(a cos C+ c cos A)
= c cos B + b cos C ....(by projection rule]
= a ...[by projection rule]
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