If sin θ = 3/5, find cos θ, given that θ is acute.

Using the Pythagorean identity, if sin θ = 3/5, then the opposite side is 3 and hypotenuse is 5, so the adjacent side = √(5² - 3²) = √(25 - 9) = √16 = 4. Therefore cos θ = adjacent/hypotenuse = 4/5. The other options arise from common errors such as inverting the ratio or using the wrong sides.