T O P I C R E V I E W 
Cortez Mos 
Posted  10/17/2007 : 23:16:16 cos(10)cos 35+ sin(10) sin 35 
4 L A T E S T R E P L I E S (Newest First) 
sahsjing 
Posted  10/18/2007 : 17:37:35 Another approach: cos(10)cos 35+ sin(10) sin 35 = sin80 cos35  cos 80 sin35 = sin(8035) = sin 45 = 2 / 2 
the_hill1962 
Posted  10/18/2007 : 07:34:54 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(AB) 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. 3510=45 So, since you problem is equivalent to cos(AB) 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

Cortez Mos 
Posted  10/17/2007 : 23:25:35 example please 
royhaas 
Posted  10/17/2007 : 23:24:30 Just plug into the formula and simplify. 