Question

cosA + cosB =2cos(A+B/2) cos(A-B/2).....PROVE

Answer 1

i hope it will help u

Answer 2

Step-by-step explanation:

We take a basic formula first  

which is  

cos(a+b)=cos a cosb-sin a sin b      — – –1

cos (a-b)= cos a cos b +sin a sin b — — -2

adding both formula

cos(a+b)=2 cos a cos b     — —3

cos a + cos b =2 cos a cos b ----------4

let supause  

a+b=A         Capital A

a-b=B          capital B

here when adding it  

  a+b=A

+a-b=B

equal

2a=(A+B)

a=(A+b)/2

and when we subtact it  

a+b=A

-(a-b)=B

2b=(A-B)

b=(A-B)/2

now  

put it in equation------4

cos A +cos B=2 cos A+b/2 cos A-B/2 hance proved