Click anywhere in the colored area to reposition the top corner of the black-outlined triangle.
When you put the top corner above the baseline of the triangle the side edges of the triangle split the blue and the red rectangle into half.
What is the area of the rectangle composed of the red and the blue one? What is the area of the black-outlined triangle?
What happens if the top corner of the triangle is not not above the baseline but in the upper left corner or the upper-right corner of the yellow rectangle? Why does the same simple formula for the area of the triangle still hold?