I believe the formula that royhaas is referring to is: cos(A−B) = cos A cos B + sin A sin B At least that is the obvious one to me from the following that you should have seen before being assigned this problem: 1. cos(A+B) = cos A cos B − sin A sin B 2. sin(A+B) = sin A cos B + cos A sin B 3. cos(A−B) = cos A cos B + sin A sin B 4. sin(A−B) = sin A cos B − cos A sin B The reason that we pick out the 3rd one is because your problem is cos(-10)cos 35+ sin(-10) sin 35 Notice if you have A=-10 and B=35 then your problem is cos(A)cos(B) + sin(A)sin(B) which is EXACTLY what the 3rd identity states that cos(A-B) is equal to. Now all you have to do is the simple problem of subtracting B from A (which in your case is -35 minus 10. -35-10=-45 So, since you problem is equivalent to cos(A-B) where A=-10 and B=35, it is cos(-10 - 35), and that is cos(-45). Now, you should already know that cos(-45) is equal to (2)/2
Interactive Math Goodies Software
using the cosine sum and difference to find values
Posted - 10/17/2007 : 23:16:16
2)/2
