Perimeter of Right Triangle

Welcome to our Perimeter of Right Triangle support page.

We will explain how to find the perimeter of a right-angled triangle and show you some worked examples.

Perimeter of Right Triangle support page

  • Perimeter is the distance around the outside of a shape.
  • It is measured as a length and can be in a range of units such as cm, inches, feet, miles, etc.

When we are trying to find the perimeter of a right triangle, there are several different ways we can do this.

Which way you select will depend on several factors, such as whether the shape is drawn to scale and whether the side lengths are given.

Option 1 - Side lengths are all given

This is the easiest example to deal with.

  • All you need to do to find the perimeter of right triangle is to add up the three side lengths.

Examples

Example 1

right triangle perimeter example 1

In the example above, we just need to add up the lengths of the three sides:

3cm + 4cm + 5cm = 12cm.

So the perimeter of right triangle is 12cm.

Option 2 - Side lengths are not given but the shape is drawn to scale.

This is another straightforward example to work out.

  • All you need to do is to use a ruler, measure the sides and add up the length of each of the sides.

Note that you will often end up with a decimal answer for your perimeter.

Option 3 - the shape is not to scale and the longest side is missing its value.

To work this out, we need to use Pythagoras' theorem to find the longest side and then add on the lengths of the other two sides.

  • Use Pythagoras' theorem to find the length of the longest side and then add on the lengths of the other two sides.

Examples

Example 1 (not drawn to scale)

right triangle perimeter example 2

Pythagoras' theorem says that the square of the two shorter sides on a right triangle is equal to the square of the longest side (the hypotenuse).

So if h is the longest side (or hypotenuse), then h2 = 62 + 32.

This gives us that h2 = 36 + 9 = 45

So h must equal √45 which is equal to 6.71 to 2dp.

The next step is to add up the lengths of all the sides.

6.71cm + 6cm + 3cm = 15.71cm

So the perimeter of the right triangle is 15.71cm to 2dp.

Example 2 (not drawn to scale)

right triangle perimeter example 3

Using Pythagoras' theorem again.

So if h is the longest side (or hypotenuse), then h2 = 42 + 42.

This gives us that h2 = 16 + 16 = 32

So h must equal √32 which is equal to 5.66 to 2dp.

The next step is to add up the lengths of all the sides.

5.66cm + 4cm + 4cm = 13.66cm

So the perimeter of the right triangle is 13.66cm to 2dp.

Option 4 - the shape is not to scale and one of the legs (shorter sides) is missing its value.

To work this out, we need to use Pythagoras' theorem to find the shorter side (or leg) that is missing and then add on the lengths of the other two sides.

  • Use Pythagoras' theorem to find the length of the leg and then add on the lengths of the other two sides.

Examples

Example 1 (not drawn to scale)

Find the perimeter of right triangle below.

right triangle perimeter example 4

Pythagoras' theorem says that the square of the two shorter sides (legs) on a right triangle is equal to the square of the longest side (the hypotenuse).

So if s is the leg that is missing, then s2 + 52 = 82.

So s2 + 25 = 64

This means that s2 = 64 - 25 = 39

So s = √39 = 6.24cm to 2dp.

The next step is to add up all the sides.

6.24cm + 5cm + 8cm = 19.24cm.

So the perimeter of the right triangle is 19.24cm to 2dp.

Example 2 (not drawn to scale)

right triangle perimeter example 5

Pythagoras' theorem says that the square of the two shorter sides on a right triangle is equal to the square of the longest side (the hypotenuse).

So if s is the leg that is missing, then s2 + 62 = 102.

So s2 + 36 = 100

This means that s2 = 100 - 36 = 64

So s = √64 = 8cm.

The next step is to add up all the sides.

8cm + 6cm + 10cm = 24cm.

So the perimeter of the right triangle is 24cm.

Perimeter of Right Triangle worksheets

If you would like to practice these skills yourself, why not try our worksheets all about finding the perimeter of a right triangle.

The first sheet involves measuring with a ruler only.

The second sheet involves using Pythagoras' theorem (whole number answers only)

The third and fourth sheets involve finding the perimeters of right triangles using Pythagoras' theorem with answers to 1dp.

More Recommended Math Resources

Take a look at some more of our resources similar to these.

More Perimeter Resources

We have a selection of perimeter help and support as well as perimeter worksheets.

The perimeter worksheets are all graded in order from easiest to hardest.

Using these sheets will help your child to:

  • work out the perimeter of a range of rectangles;
  • find the perimeter of rectilinear shapes.

Area Worksheets

The area sheets are all graded in order from easiest to hardest.

Using these sheets will help your child to:

  • work out the areas of a range of rectangles;
  • find the area of rectilinear shapes.

All the sheets in this section support Elementary Math Benchmarks.

3rd Grade Measurement Worksheets

Here is our selection of measurement worksheets for 3rd graders.

These sheets involve reading scales going up in halves, ones, fives, tens, fifties and hundreds to find the length, weight or liquid capacity.

Using these sheets will help children to consolidate their counting and place value as well as learning to read and estimate using a simple scale.

Online Perimeter Practice

Another way to learn about 'What does perimeter mean' is through exploring shapes.

This online area and perimeter activity by toytheater.com will let you explore 'what does perimeter mean' through creating your own shapes.

The link below will open in a new browser window.

 

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