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 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(80-35)= 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 BAt 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 B2. sin(A+B) = sin A cos B + cos A sin B3. cos(A−B) = cos A cos B + sin A sin B4. sin(A−B) = sin A cos B − cos A sin BThe reason that we pick out the 3rd one is because your problem iscos(-10)cos 35+ sin(-10) sin 35Notice if you have A=-10 and B=35then your problem iscos(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=-45So, 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 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.

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