Because otherwise you would have found the area of the rectangle/square (the rectangle/square being if you put two of the triangles together).
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They are both polygon. If you cut a rectangle diagonally, you will have two triangles. And if you want to figure out the area of a right triangle, you can treat it as a rectangle and then cut your calculation in half to get the correct answer.
the area of a triangle is half of the base times the height the area of a triangle is half of the base times the height
A triangle
The area of qa triangle is always half of the area of a rectangle with the same dimensions
It's half the area of a rectangle