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Triangle Calculator
Enter the three side lengths — area, perimeter and angles update instantly.
Formula
Area = √(s(s−a)(s−b)(s−c)) where s = (a+b+c)/2 | Angles via Law of Cosines: cos A = (b²+c²−a²)/2bc
Example
Sides 3, 4, 5: s = 6, Area = √(6·3·2·1) = 6, angles 36.87° / 53.13° / 90° (right triangle).
FAQ
How do I find a triangle area from 3 sides?
Use Heron's formula: compute the semi-perimeter s = (a+b+c)/2, then Area = √(s(s−a)(s−b)(s−c)).
What is the triangle inequality?
Any side must be shorter than the sum of the other two. If not, the three lengths cannot form a triangle.