sin(a-b)=sina cosb-cosa sinb
This is a trig identity.
a=phi b=theta
Phi and theta are Greek letters commonly used to name specific angles.
i dont understand anything!!!
Maybe you've come to the wrong place for help. Volunteers are here to help guide people through specific issues. If you are not sufficiently familiar with the material to be able to discuss it, then you might need classroom help.
You have not asked any specific questions. I don't even know if what you've submitted comprises an actual exercise.
If you want to discuss an actual exercise, then please provide all of the given information, and then explain what you already know about the exercise. Show whatever work that you can. Tell us why you're stuck.
Only then will we be in a position to determine where to start helping you.
Cheers ~ Mark 
