Free · Mathematical relations and geometry
Four-Side Perimeter and Missing Side
Add four side lengths for perimeter or recover a positive missing side from the other three and perimeter.
- Formula & worked example
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Calculator inputs
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How to use this calculator
- Enter the known values in the units shown. Results update as you type.
- Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
- Use the worked example to check the method. Reset restores the starting fields.
Private by default
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Formula and method
Sides 8, 5, 4 and 5 have perimeter 22. With perimeter 22 and known sides 8, 5 and 4, the missing side is 5.
P=a+b+c+d
Worked example
Enter these known values and leave the other values blank.
- a
- 8 cm
- b
- 5 cm
- c
- 4 cm
- d
- 5 cm
- Perimeter (P)
- 22 cm
Assumptions and limitations
- The relation sums four positive side lengths. It does not verify whether those lengths form a trapezoid.
- A missing side equals the positive remainder after subtracting the known three sides from the perimeter.
- At least one pair of opposite sides must be parallel for the shape to be a trapezoid; this perimeter relation alone does not prove that condition.
Common questions
How can I solve for a missing quantity?
Leave the unknown control empty and enter the quantities needed by the selected relation. Read the assumptions for any branch or domain restrictions.
How should I interpret the worked example?
It is an exact mathematical model with independently checked numbers. Display rounding does not change the values used for subsequent calculations.
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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