If 
, prove that 
cosx+cosy = 
--------- i
sinx+siny = 
----------ii
dividing ii by I we get,
= 
= 
= 
Using the formula,
sinA + sinB = 2sin
cos
cosA + cosB = 2cos
cos
1
If , prove that
cosx+cosy = --------- i
sinx+siny = ----------ii
dividing ii by I we get,
=
=
=
Using the formula,
sinA + sinB = 2sin cos
cosA + cosB = 2coscos