To find the height \( h \) of the square pyramid using the Pythagorean theorem, we can visualize a right triangle formed by the height, half the base edge, and the slant height.
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The base edge is given as 40 feet, so half of this is: \[ \frac{40}{2} = 20 \text{ feet} \]
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The slant height is given as 25 feet.
Now we have a right triangle where:
- One leg is the height \( h \).
- The other leg is half the base edge, which is 20 feet.
- The hypotenuse is the slant height, which is 25 feet.
According to the Pythagorean theorem, we have: \[ h^2 + (20)^2 = (25)^2 \]
Calculating \( (20)^2 \) and \( (25)^2 \): \[ h^2 + 400 = 625 \]
Now, solve for \( h^2 \): \[ h^2 = 625 - 400 \] \[ h^2 = 225 \]
Taking the square root of both sides to find \( h \): \[ h = \sqrt{225} = 15 \text{ feet} \]
Rounding to the nearest tenth, the height \( h \) of the square pyramid is: \[ \boxed{15.0 \text{ ft.}} \]