Free · Algebra and geometric models

Check Whether Three Sides Form a Triangle

Classify positive side lengths using all three strict triangle inequalities.

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

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Formula and method

Sides 3, 4 and 5 form a nondegenerate triangle: each pair sums to more than the remaining side.

a+b>c; a+c>b; b+c>a

Worked example

Enter these known values and leave the other values blank.

Side length a
3 cm
Side length b
4 cm
Side length c
5 cm
Result
These positive sides form a nondegenerate triangle.

Assumptions and limitations

  • The sides are positive and share a common length scale.
  • Equality describes a degenerate straight segment, so it fails the strict triangle test.

Common questions

What must I enter?

Choose the mode when available and enter its independent known quantities. Leave an unknown editable quantity empty to use a supported inverse.

How should I use the result?

Interpret the numbers within the stated mathematical assumptions. Available unit selectors convert quantities; display rounding does not replace the internal calculation.

References

The calculation equations, inverse formulas, units, and input rules were imported from this source. Bookify provides the interface and equation solver.

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