The interior angles are those on the inside of the triangle.
Try this Drag the orange dots on each vertex
A,B or C to reshape the triangle. Notice that the interior angles always add to 180°
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The interior angles of a triangle always add up to 180°. Because of this, only one of the angles can be 90° or more.
In a right triangle, since one angle is always 90°, the other two must always add up to 90°
A triangle is simply a polygon that has 3 sides. See interior angles of a polygon
for the properties of the interior angles of a polygon with any number of sides.
Properties
The interior angles of a triangle always add up to 180°
Because the interior angles always add to 180°, every angle must be less than 180°