Question

Angel ABC dwara sidh ki jiye ki a=bcosc+ccosb

Answer 1

Answer:

To prove in △ABC

Main formula cosA=

2bc

b

2

+c

2

−a

2

⇒a=bcosC+ccosB

This is called projection formula :

In the RHS we have :-

bcosc+ccosb

we know cosine formula

⇒cosA=

2bc

b

2

+c

2

−b

2

using cosine formula,

=b

2ab

(b

2

+a

2

−c

2

)

+c

2ac

(c

2

+a

2

−b

2

)

=

2a

b

2

+a

2

−c

2

+

2a

c

2

+a

2

−b

2

=

2a

2a

2

=a

on LHS we have a LHS = RHS

Hence, solved.