Answer:
39√2 feet
Explanation:
You want the length of the diagonal of a square that is 39 feet on a side.
Diagonal
The Pythagorean theorem can be used to find the length of the diagonal. It tells you the length of the hypotenuse of a right triangle with legs 39 ft will be ...
d² = 39² +39²
d² = 39²(1 +1) = 39²·2
d = √(39²·2) = 39√2
The length of the diagonal is 39√2 feet.
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Additional comment
As you can tell from this answer, the length of the diagonal of any square is √2 times the length one side. Knowing this, you can write down the answer without any further effort.