EASA / CAR Part-66 · Module 1 · 1.3 Geometry · Cat B1/B2
Objective: real parts are rarely one clean shape. Build a composite section from a rectangle, a triangle and a semicircle, and see the total area come from adding the pieces, or from subtracting a hole.
A trapezium is the average of its two parallel sides times the height. ½(a + b) is that average width, so if the top and base are equal you get a rectangle back, and if the top goes to zero you get a triangle. One formula covers all three.
Break a composite shape into pieces you already know. A bracket or a web is a rectangle plus a triangle, or a rectangle with a lightening hole cut out. Work out each simple piece, then add or subtract. There is no separate formula to memorise.
Holes subtract, and they matter more than they look. Doubling the diameter of a lightening hole takes out four times the area, because a circle's area goes with the square of the radius, which is why the percentage removed climbs so fast.