Mathematics

This is about finding a trigonometric ratio in a right triangle. The key insight is that $\cos D$ means the ratio of the adjacent side to the hypotenuse from angle $D$'s perspective. First, use the Pythagorean theorem to find the hypotenuse $DF$, then identify which side is actually next to angle $D$ — that's $DE$, not the one opposite to it.

This is about finding a trigonometric ratio in a right triangle. The key insight is that $\cos D$ means the ratio of the adjacent side to the hypotenuse from angle $D$'s perspective. First, use the Pythagorean theorem to find the hypotenuse $DF$, then identify which side is actually next to angle $D$ — that's $DE$, not the one opposite to it.