To find the angle measure of a triangle using trigonometric ratios, you will use the inverse of a ratio. You are asked to use the cosine ratio (Adjacent/Hypotenuse) which is cos 5/13. All you have to do is add an inverse to get cos^-1 5/13. You could also write it as 1/sin 12/13 because the expression 1/sin(x) is equivalent to cos^-1 5/13.
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