Answer: 12/13
In other words, 12 goes in the top box and 13 goes in the bottom. The fraction slash sign is not part of either box.
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Step-by-step explanation:
Cosine is the ratio of adjacent over hypotenuse.
cos(angle) = adjacent/hypotenuse
For the reference angle X, the adjacent leg is 36 units long. It's the leg closest or touching angle X.
The hypotenuse is always the longest side. It is always opposite the 90 degree angle. The hypotenuse in this case is 39 units.
Therefore,
cos(X) = 36/39 = (12*3)/(13*3) = 12/13