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In any DeltaABC, 2 (bc cos A + ca cos B ...

In any `DeltaABC`, 2 (bc cos A + ca cos B + ab cos C) =

A

`a^2+b^2+c^2`

B

`a^2-b^2+c^2`

C

`a^2+b^2-c^2`

D

`a^2-b^2-c^2`

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A
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