Answer: 3/5
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Step-by-step explanation:
M = 90 and the side opposite this is LK = 5 since "LK" doesn't have "M" anywhere in the letter sequence. The side LK is the hypotenuse. The hypotenuse is always the longest side and always opposite the right angle.
The reference angle is angle K. Opposite this angle is ML = 4. Note how ML has no mention of K. The adjacent side to angle K is KM = 3 as it's the only thing left and we have a mention of K now.
The cosine ratio is therefore...
cos(angle) = adjacent/hypotenuse
cos(K) = KM/LK
cos(K) = 3/5