The ratio is 3:4:5.
Cosine by definition, is adjacent divided by hypotenuse, meaning that the value of the hypotenuse is 5. Then, we are given two other sides that are adjacent to angles A and B, which are 4 and 3 respectively.
This means that there are side lengths 3, 4, and 5, meaning that the ratio of the sides are 3:4:5.
Hope this helps.
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