GuidesGeometry and trigonometry

Special Right Triangles: 45-45-90 and 30-60-90

Know two ratios, and you can find missing sides quickly.

Worked examples

A 45-45-90 triangle has a leg of 5. Find the hypotenuse.

Hypotenuse

A 30-60-90 triangle has a short leg of 4. Find the other sides.

Long leg

Hypotenuse

Common mistake

Putting across from the angle.

The shortest side is across from the smallest angle. So goes across from , and goes across from .

Try it yourself

  1. A 45-45-90 triangle has a leg of 7. Find the hypotenuse.
    Show answer

  2. A 30-60-90 triangle has a hypotenuse of 10. Find both legs.
    Show answer

    and

  3. A 45-45-90 triangle has a hypotenuse of . Find a leg.
    Show answer

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