Answer:
[tex]15\sqrt{2}\ ft[/tex] ≈ [tex]21.21\ ft[/tex]
Step-by-step explanation:
Observe the figure attached.
You need to use the following Trigonometric function in order to find the length of the diagonal "d":
[tex]cos\alpha=\frac{adjacent}{hypotenuse}[/tex]
For this case:
[tex]\alpha=45\°\\adjacent=15\\ hypotenuse=d[/tex]
Therefore, you need to substitute these values into the Trigonometric function and solve for "d":
[tex]cos(45\°)=\frac{15}{d}\\\\d*cos(45\°)=15\\\\d=\frac{15}{cos(45\°)}[/tex]
[tex]d=15\sqrt{2}\ ft[/tex] ≈ [tex]21.21\ ft[/tex]