Picture 1: The diagonal line in the 5 x 13 picture has been blurred (too thick) deliberately, which increases the area mistakenly. This is because the slopes of the triangle (3/8 for A and B) is not equal to the slope of the quadrangle (2/5 for C and D)
Picture 2: the top triangle has a "sunken-in" tangent line, while the bottom one has a bulged tangent line. Same reason for the two triangles, the slope ratio is 2/5 to 3/8.www.ddhw.com