Hello JadeKiaraThe triangle's area is one-half of the area of the rectangle.

The height of the triangle is the length of the line from E to the base, DC produced. And this is equal in length to BC and to AD.

So the area of the triangle is 1/2 base x height, which is 1/2 DC x AD. And of course, this is 1/2 the area of the rectangle.

Grandad