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**1. The problem statement, all variables and given/known data**

find the exact value of cos(arctan(2)+arctan(3))

**3. The attempt at a solution**

I know that arctan(2) is equivalent to tanx=2 and arctan(3) is equal to tanx=3

I was thinking perhaps sinx/cosx=2 and 3 but I am not sure what numbers to use. I'm kind of weak in my trig rules =( can someone explain to me how I would go about this problem? I know the answer is -sqrt2/2 but I want to know HOW to do it.