By using trigonometry, we conclude that the horizontal distance is 785.15ft
We can think of this as a right triangle.
Where the height of the observation deck is one cathetus and the horizontal distance is the other cathetus,
We know that the depression angle when looking from above is 42°. If we step on this angle, the adjacent cathetus is the height of the deck, then the horizontal distance is the opposite cathetus.
Then we can use the relation:
tan(a) = (opposite cathteus)/(adjacent cathetus).
Replacing what we know:
tan(42°) = (distance)/(872ft)
tan(42°)*872ft = distance = 785.15ft
The horizontal distance is 785.15ft
If you want to learn more about right triangles:
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