What is the shorter side in a triangle?

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GCSE

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Pythagoras' theorem

Pythagoras’ theorem can be used to calculate the length of any side in a right-angled triangle. Pythagoras’ theorem can be applied to solve 3-dimensional problems.

Part of

Maths

Geometry and measure

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Calculating the length of one of the shorter sides

Rearrange Pythagoras' theorem to calculate the length of one of the shorter sides.

\(a^2 + b^2 = c^2\) calculates the length of the longest side.

To calculate the length of one of the shorter sides, rearrange the formula to make \(a^2\) or \(b^2\) the subject.

\(a^2 = c^2 - b^2\) or \(b^2 = c^2 - a^2\)

Then take the square root to calculate the length \(a\) or \(b\).

Example

Calculate the length AB.

Right angle triangle (ABC) (sides: 10cm x 8cm x unknown)

\[a^2 + b^2 = c^2\]

\[8^2 + b^2 = 10^2\]

Rearrange the formula to make \(b^2\) the subject:

\[b^2 = 10^2 - 8^2\]

\[b^2 = 36\]

\[b = \sqrt{36}\]

\[b = 6~\text{cm}\]

The length AB is 6 cm.

Question

Calculate the length AO. Give the answer to one decimal place.

Right angle triangle (ABC) (sides: sq root 17cm x 3cm x unknown)

\[a^2 + b^2 = c^2\]

\[3^2 + b^2 = (\sqrt{17})^2\]

Rearrange the formula to make \(b^2\) the subject.

\[b^2 = (\sqrt{17})^2 - 3^2\]

\[b^2 = 17 - 9\]

\[b^2 = 8\]

\[b = \sqrt{8}\]

\[b = 2.8~\text{cm}\]

The length AO is 2.8 cm.

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What is the shortest side of a triangle?

The shortest side of a triangle is always opposite the smallest angle. For example, in ∆ABC below, side BC is the shortest side and is opposite angle A.

What is the 3 4 5 Triangle rule?

The 3:4:5 triangle is the best way I know to determine with absolutely certainty that an angle is 90 degrees. This rule says that if one side of a triangle measures 3 and the adjacent side measures 4, then the diagonal between those two points must measure 5 in order for it to be a right triangle.