The first picture looks like a triangle at first glance, but it is not. The hypotenuse is not a straight line, which can be proved.

The ratio of two right-angled sides of the blue triangle above is 2: 5.

If the whole figure is a triangle, it should be similar to a blue triangle, but the right angle ratio of the whole figure is 5: 13.

So the hypotenuses of the blue triangle and the red triangle are not in a straight line, but the slopes are similar. Form a concave quadrangle

On the other hand, the second picture is a convex quadrilateral. The landlord wants to know for himself.

Or the sum of the areas of each thumbnail can be calculated.

Blue: 0.5 * 2 * 5 = 5

Yellow: 7

Green: 7

Red: 0.5 * 3 * 8 = 12

The total is 32.

And if calculated by triangle: 0.5 * 13 * 5 = 32.5.

The difference of 0.5 is the depressed area.

The second picture is just convex by 0.5, so the area difference between the two pictures is 1.