Author 
Topic 

Cortez Mos
Junior Member
USA
3 Posts 
Posted  10/17/2007 : 23:16:16

cos(10)cos 35+ sin(10) sin 35 


royhaas
Moderator
USA
3059 Posts 
Posted  10/17/2007 : 23:24:30

Just plug into the formula and simplify. 


Cortez Mos
Junior Member
USA
3 Posts 
Posted  10/17/2007 : 23:25:35

example please 


the_hill1962
Advanced Member
USA
1468 Posts 
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



sahsjing
Advanced Member
USA
2399 Posts 
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 



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