# Can't make sense of this trig identity

1. Sep 10, 2008

### banfill_89

1. The problem statement, all variables and given/known data
im in first year differential calculas and i have no idea what my prof wrote down...i just copied it and thought ide figure it out later. but i cant fore the life of me.

2. Relevant equations
the identitie that he wrote is:

sinC+sinB=2Sin (C+D)/2 cos (c-D)/2

3. The attempt at a solution

2. Sep 10, 2008

### banfill_89

Re: identities!!!!

side note ) x+y = c
x-y = b

3. Sep 10, 2008

### banfill_89

Re: identities!!!!

i understand where the sin C + sin B, its the (c+d)/2 that has me confused.........these identities are just another way of writing the identities derived from adding and subtracting the addition and subtraction identities

4. Sep 10, 2008

### Dick

Re: identities!!!!

Change the 'D' to 'B'. Then it works. Sure, it's derived from addition and subtraction identities. Basically everything is.

5. Sep 10, 2008

### banfill_89

Re: identities!!!!

where did the (c+b)/2 come from.......i know they subbed that in for x....but why?

6. Sep 10, 2008

### Dick

Re: identities!!!!

I don't know, you didn't give us the whole problem. I do know sin(c)+sin(b)=2*sin((c+b)/2)*cos((c-b)/2).