Answer:
y ≈ 240.3 ft
Explanation:
The mnemonic SOH CAH TOA reminds you that the relation between an angle and the sides adjacent and opposite is ...
Tan = Opposite/Adjacent
In this triangle, the interior angle at lower right is the same as the one marked at upper left: 31°. y is the side opposite that angle, and 400 ft is the side adjacent. Then the relation is ...
tan(31°) = y/(400 ft)
Multiplying by 400 ft gives ...
y = (400 ft)·tan(31°) ≈ 240.3 ft
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The triangle interior angle at lower right and the angle marked 31° are "alternate interior angles" relative to the transversal marked x and the (parallel) horizontal lines in the figure. Alternate interior angles always have the same measure.
In geometry problems like this one, it means the angle of elevation (above the horizontal) is equal to the angle of depression (below the horizontal).