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Skills for Learning: Maths & Stats - Trigonometry

Learning Outcomes

If you work through this section you should be able to:

  • Understand Pythagoras’ theorem and its usefulness.
  • Find the length of a side of a right-angled triangle.
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The Pythagoras’ theorem relates to the lengths of the three sides of a right-angled triangle. The Pythagoras’ theorem states that the square on the longest side of a right-angled triangle is equal to the sum of the squares on the two shorter sides of the triangle.

The Pythagoras’ theorem can be used to find the length of the third side of the right-angled triangle if the lengths of the other two sides are provided. The Pythagoras’ theorem can also be used to determine whether a triangle has a right angle (90°).

Formula

Diagram of a triangle demonstrating Pythagoras' theorem.

The square on the hypotenuse of a right-angled triangle is equal to the sum of the squares on the other two sides.

BC2 = AB2 +AC2

Example 1

Given one side and the hypotenuse of a right-angled triangle as 5.6cm and 12.3cm respectively, find the length of the other side.

5.62 + x2 = 12.32

31.36 + x2 = 151.29

x2 = 119.93

x = 10.95cm

Example 2

Given two sides of a right-angled triangle as 5cm and 7cm, find the length of the hypotenuse.

x2 = 52 + 72

x2 = 25 + 49

x2 = 74

x = 8.6cm

Example 3

Check whether the triangle with sides 9m, 40m and 41m is a right-angled triangle.

BC2 = 412 = 1681

AB2 + AC2 = 92 + 402 = 81 + 1600 = 1681

BC2 = AB2 + AC2

The triangle satisfies Pythagoras’ theorem and so is a right-angled triangle.

Activity

You can download a version of this Pythagoras’ theorem activity in Word format:

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