Diagonal AC must be 20 feet so that ∠B will be 90°.
According to asked question, buck is creating a horse coral by designing in such a way that corners will be at 90°.
So according to question we have to make ∠B as 90°.
So quadrilateral ABCD becomes a rectangular.
Therefore ΔABC will be a right-angle triangle in which we can use Pythagoras theorem to measure the diagonal AC.
The Pythagoras Theorem says that in a right-angle triangle, the square of the hypotenuse is the sum of square of the other two sides of the right-angle triangle.
In the right-angle tringle ΔABC,
AC²=AB²+BC²
⇒ AC²=16²+12²
⇒ AC²=256+144=400
⇒AC=√400
⇒AC=20 feet
Therefore, Diagonal AC must be 20 feet so that ∠B will be 90°.
Learn more about Pythagoras Theorem
Here: https://brainly.com/question/343682
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