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Perimeter of a triangle worksheets

A triangle's perimeter is the distance around it: add the three sides. These free printable worksheets label every edge, including the slanted ones, so all three numbers count toward the answer.

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Perimeter of a triangle
Perimeter of a triangle worksheet — Sides to 35 cm, free printable with answer key

The answer key prints on a separate page.

What's on this sheet

This is the triangle taken out of the mixed perimeter page, which rotates through triangles, parallelograms, trapezoids, and rectangles. On its own it is a clean three-number addition, which is where a child starting perimeter should begin.

Why this sheet works

Every side carries its own number — the opposite of the area sheets, where slant sides are deliberately left blank. Here a blank edge would make the figure unanswerable, so all three are printed, and the triangle is drawn to match them: sides of 14, 13, and 17 are drawn as exactly that triangle.

How this one works

One question from this sheet, worked through a step at a time.

Perimeter of a 9-11-14 triangle

sides:91114
sides:91114
sides:91114
9 + 11 + 14=34 cm
  1. Perimeter is the distance round the OUTSIDE — the length of the fence, not the size of the field. That is the whole difference between this and area, and it is where the usual wrong answer comes from.
  2. So there is nothing to work out — just walk round the edge and add: 9 + 11 + 14.
  3. 9 + 11 + 14 = 34 cm. Note the unit: cm, not square cm — a fence is a length, and a length never gets squared.

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