Question 849013
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Let's assume that the picture you attached looked something like this:

<img src=http://www.tiem.utk.edu/~gross/bioed/bealsmodules/triangle1.gif />
 
Instead of {{{theta}}} let's call it A.

We think of our good ol' friend chief  SOH-CAH-TOA. Where:

Sin = opp/hyp
Cos = adj/hyp
Tan = opp/adj

So for sin(A), locate where A is. Indicate the side OPPOSITE of A, that's our numerator. Then find the HYPOTENUSE, that's our denominator. This is always the side opposite of our right angle and it is the longest side.

So for cos(A), locate where A is. Indicate the side ADJACENT of A, that's our numerator. Then find the HYPOTENUSE, that's our denominator.