L4 · Perimeter: the sum of the side lengths of a polygon

Learning goal

“I can find the perimeter of a polygon by adding the lengths of ALL its sides - and when some sides are equal, I can use multiplication to add them faster.”

Perimeter — the distance all the way around a polygon; the sum of the lengths of its sides.

Polygon — a closed shape with straight sides, like a triangle, a square, or a pentagon.

Side — one straight edge of a polygon.

Sum — the total you get when you add numbers together.

Worked example

A triangle has sides 5 cm, 7 cm, and 6 cm. Find its perimeter. Step 1: Perimeter means walk the whole border and add every side. Step 2: 5 + 7 + 6 = 18 cm. Second shape: a rectangle is 8 cm by 4 cm. Its four sides are 8, 4, 8, 4. Step 3: 8 + 4 + 8 + 4 = 24 cm. Say it plainly: a rectangle has FOUR sides - the two lengths you are given each appear twice. Never stop after two sides.

Check yourself — explain it

A perimeter is found by...

Paired problem — example still visible

A triangle has sides 3 cm, 5 cm, and 4 cm. Perimeter? -> 12 cm (3 + 5 + 4)

Faded completion — last step is yours

"A rectangle is 6 cm by 3 cm: P = 6 + 3 + 6 + ___ = ___ cm" -> 3; 18

Ready to practise?

10 scored questions with a 3-rung hint ladder. Mastery needs 8/10 plus both gate questions.

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