How to find a right angle triangle

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GCSE

CCEA

Pythagoras' theorem

Pythagoras' theorem is used to find the length of a side in a right-angled triangle.

Part of

Maths

Geometry and measure

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Identifying right-angled triangles using Pythagoras' theorem

It is possible to determine if a triangle contains a right angle using Pythagoras' theorem.

If the squares of the two shorter sides add up to the square of the hypotenuse, the triangle contains a right angle.

Example

Does the triangle ABC contain a right angle?

Triangle (ABC) with sides 5cmx6cmx8cm

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

\[5^2 + 6^2 = c^2\]

\[61 = c^2\]

The hypotenuse of the triangle is 8.

\[8^2 = 64\]

61 does not equal 64. Therefore, the triangle does not contain a right angle.

Question

Does the triangle PQR contain a right angle?

Triangle (QPR) with sides measuring 5cm, 13cm & 12cm

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

\[5^2 + 12^2 = c^2\]

\[169 = c^2\]

The hypotenuse of the triangle is 13.

\[13^2 = 169\]

169 is equal to \(c^2\). Therefore, the triangle does contain a right angle.

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What is the formula for a right angle triangle?

In a right triangle with cathetus a and b and with hypotenuse c , Pythagoras' theorem states that: a² + b² = c² . To solve for c , take the square root of both sides to get c = √(b²+a²) . This extension of the Pythagorean theorem can be considered as a "hypotenuse formula".

How do you find an angle in a right triangle with two sides?

Example.
Step 1 The two sides we know are Adjacent (6,750) and Hypotenuse (8,100)..
Step 2 SOHCAHTOA tells us we must use Cosine..
Step 3 Calculate Adjacent / Hypotenuse = 6,750/8,100 = 0.8333..
Step 4 Find the angle from your calculator using cos-1 of 0.8333:.