2. Acute angle refers to an angle greater than 0 and less than 90. The acute angle is less than the right angle (90 degrees).

3. The obtuse angle refers to an angle greater than 90 and less than180. The obtuse angle is greater than the right angle (90 degrees) and less than the right angle (180 degrees).

The way to judge whether an angle is a right angle, an obtuse angle or an acute angle is to use a right triangle, so that the vertex of the angle coincides with the vertex of the right triangle, one side coincides with one side of the right triangle, and then look at the other side.

If the other side coincides with the other side of a right triangle, the angle is a right angle.

If the other side is within the other side of a right triangle, the angle is acute.

If the other side is outside the other side of the right triangle, the angle is obtuse.

Extended data:

A triangle has at most 3 acute angles and at least 2 acute angles. A triangle with three acute angles is called an acute triangle. In an acute triangle, every internal angle is acute, and the sum of any two internal angles is greater than the right angle; Each side is sandwiched between its adjacent side and the product sum quotient of cosine of their included angle, and the sum of squares of any two sides is greater than the square of the third side.

A triangle has at most 1 right angles. Right triangle, there is a right triangle, there are two kinds of ordinary right triangle and isosceles right triangle.

A triangle has at most 1 obtuse angles.

References:

Baidu encyclopedia-acute angle

Baidu encyclopedia-right angle

Baidu encyclopedia-obtuse angle