Composite figures (also called irregular shapes) are made by combining simpler shapes. The trick is to decompose them into rectangles and triangles, track the outer boundary carefully, and keep units consistent.
You can draw helper lines to split a shape into simpler blocks, but you should not add those lines unless they lie directly on the outer edge.
Area calculations are easiest by decomposing the shape into rectangles and triangles, or by taking a large simple shape and subtracting missing piece areas.
Perimeter measures the true distance around the perimeter of the shape. For irregular figures, a common error is adding interior split lines.
Did you just calculate perimeter by doubling the sum of two adjacent lengths like a standard rectangle? Recheck! An irregular figure likely has indentations or missing corners that drastically change outer lengths.
Split the overall irregular shape directly into known chunks (Triangles, Rectangles) and sum their areas together.
Construct a larger regular encompassing boundary to simplify math, then subtract the portions that don't exist in the real target.
Perimeter uses length units (cm, m). Area uses square units (cm², m²). Remember , not .
Suppose an L-shape can be enclosed inside a standard bounding box, with a rectangular section removed exactly from a single corner.
Area =
Area = square units.
Walk fully around the perimeter and add the lengths one by one. If you subtract a shape from a corner, the outer perimeter interestingly may match exactly the perimeter of the un-removed shape, but it's much safer to add the lines explicitly!