Answer :


Note: In any ΔABC we define ‘a’ as the length of the side opposite to A, ‘b’ as the length of the side opposite to B and ‘c’ as the length of the side opposite to C.


Key point to solve the problem:


Idea of projection Formula:


• c = a cos B + b cos A


• b = c cos A + a cos C


• a = c cos B + b cos C


As we have to prove:



We can observe that we can get terms c – b cos A and b – c cos A from projection formula


from projection formula we have-


c = a cos B + b cos A


c – b cos A = a cos B …..eqn 1


Also,


b = c cos A + a cos C


b – c cos A = a cos C ……eqn 2


Dividing eqn 1 by eqn 2, we have-



Hence proved.


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