Try this Drag the orange dots on each vertex to reshape the triangle.
If any angle becomes 90 degrees or more, it is not an acute triangle.

In any triangle, two of the interior angles are always
acute (less than 90 degrees)^{*},
so there are three possibilities for the third angle:

- Less than 90° - all three angles are acute and so the triangle is acute.
- Exactly 90° - it is a right triangle
- Greater than 90° (obtuse): the triangle is an obtuse triangle

(Prove it to yourself - reshape the triangle above and try to get two angles to be greater than 90°).

- Triangle definition
- Hypotenuse
- Triangle interior angles
- Triangle exterior angles
- Triangle exterior angle theorem
- Pythagorean Theorem
- Proof of the Pythagorean Theorem
- Pythagorean triples
- Triangle circumcircle
- Triangle incircle
- Triangle medians
- Triangle altitudes
- Midsegment of a triangle
- Triangle inequality
- Side / angle relationship

- Perimeter of a triangle
- Area of a triangle
- Heron's formula
- Area of an equilateral triangle
- Area by the "side angle side" method
- Area of a triangle with fixed perimeter

- Right triangle
- Isosceles triangle
- Scalene triangle
- Equilateral triangle
- Equiangular triangle
- Obtuse triangle
- Acute triangle
- 3-4-5 triangle
- 30-60-90 triangle
- 45-45-90 triangle

- Incenter of a triangle
- Circumcenter of a triangle
- Centroid of a triangle
- Orthocenter of a triangle
- Euler line

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All rights reserved