4. prove or disprove:

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

sin(a+b)=cosaxsinb+cosbxsina

sin(a-b)=sinaxcosb- cosaxsinb

sin(a+b)+sin(a-b)=sinaxcosb+cosbxsina

=2sinaxcosb

(a+b)=A ---a=(A+B)/2

(a-b)=B ---b=(A-B)/2

So, sinA+sinB=2xsin(A+B)/2cos(A-B)/2