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With usual notations prove that 2 (bc cos A +ac cos B + ab cos C = a^2 + b^2 +c^2

With usual notations prove that 2 (bc cos A +ac cos B + ab cos C = a^2 + b^2 +c^2

Grade:11

1 Answers

Arun
25750 Points
6 years ago
Dear student
 
cos A = b² + c² -a²/ 2bc
 
Hencw
2bc cos A = b² +c² -a²
similarly
2bc cos B = a² + c² -b²
&
2bc cos C = a² + b² -c²
 
Now add all the three equations, you will get the desired result.
 
Regards
Arun(askIITians forum expert) 

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