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In Δ ABC \((a – b)^2 cos^2 \frac{c}{2} + (a + b)^2 sin^2 \frac{c}{2}=\) 

A) b2 B) c2 C) a2 D) a2 + b2 + c2

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Correct answer is C2

Instead of pythagoras You can use this formulas 

cos(C) = \(a^2 + b^2 − c^2\over 2ab\)

 

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