Explanation:
remember the original trigonometric triangle inside the norm circle (radius = 1).
sine is the up/down distance from the triangle baseline or corresponding circle diameter.
cosine is the left/right distance from the center of the circle (and the point of the angle).
for larger triangles and circles all these function results need to be multiplied by the actual radius (which we skipped for the norm circle, as a multiplication by 1 is not changing anything).
when you look at the triangle with K representing the angle, we have 10 a the cosine value, 24 as the sine value and 26 as the radius.
so,
10 = cos(K) × 26
cos(K) = 10/26 = 5/13