Answer: 15/17
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Step-by-step explanation:
Cosine of an angle is the ratio of the adjacent over the hypotenuse.
For reference angle A, the adjacent leg or closest leg is side AB = 30.
The hypotenuse is always the longest side. The longest side of a right triangle is always opposite the largest angle (the 90 degree angle). So the hypotenuse is AC = 34.
We then say:
cos(angle) = adjacent/hypotenuse
cos(A) = AB/AC
cos(A) = 30/34
cos(A) = (2*15)/(2*17)
cos(A) = 15/17