How to find the area of the 45-45-90 triangle with a hypotenuse of 24?

The area of a triangle is 1/2(base x height) .

The two legs of this triangle are equal.  If you position it properly,
one of them is the base, and the other one is the height.

Since it's a right triangle,    A² + B² = C²

But  A = B  and  C = 24 .

2A² = (24)²

A² = (24)² / 2

But since  A=B,  A² is also (base x height) of the triangle.
Area is just 1/2 of it.

Area = 1/2(24²/2) = (24)²/4 = (24/2)² = (12)² = 144 .