Grade 8: Measurement

Warm up with a few quick measurement warm-ups on the board while the class settles, so every child starts thinking about the skill.

A composite shape is made of two or more simple shapes joined or cut from each other. This unit covers finding its area by adding or subtracting the simple parts, and its perimeter by tracing the outer boundary carefully. Model the method clearly, thinking aloud:

• Show how to split an L-shape (or other composite shape) into two rectangles, marking the split clearly on a diagram.
• Practise the 'add the parts' method for shapes made of joined rectangles, and the 'subtract the cut-out' method for a notch removed from a larger shape.
• For perimeter, trace the outer boundary of the composite shape step by step, not just adding the original rectangle's sides.
• Demonstrate why cutting a rectangular notch from a corner leaves the perimeter unchanged, using a labelled example.
• Mix area and perimeter questions on the same composite shapes so students practise choosing the right approach for each.

Work this through step by step on the board, then have the class talk you through a second one.

• An L-shaped room is a 12 by 15 rectangle with a 5 by 6 rectangle cut from one corner
• Area = 12 x 15 - 5 x 6 = 180 - 30 = 150 square units

Do the first few questions of the practice worksheet together, one child explaining each step. Check for understanding before releasing the class to work alone.

Students complete the practice worksheet independently while you circulate and support.

Circulate and look for these, they are the usual sticking points:

• Forgetting to subtract the cut-out rectangle's area (or add the joined rectangle's area) rather than treating the shape as one simple rectangle.
• Calculating the perimeter as if it were a simple rectangle's, ignoring the extra step edges of the notch.
• Splitting a composite shape into parts that overlap or leave a gap, so the areas do not add up correctly.
• Mixing up which two dimensions belong to the cut-out piece versus the main rectangle.

Pull the class back together. Ask one child to explain measurement in their own words, pose a single check question everyone answers on a mini whiteboard, and name what you will build on next lesson.

Use the independent worksheet as the evidence. A child who can complete it accurately and explain one answer has met the objective; anyone who cannot needs the easier level and a short reteach next session.