One day, a doctor of mathematics heard a triangle and a quadrilateral arguing: whether it is stable or not, listen to what they say.

Triangle said: "I am the most stable, because I am firm and not easy to deform, so I am popular, unlike your quadrangle, I have no firm position."

Quadrilateral said: "I am flexible and retractable." These characteristics of mine are many times better than your unchanging triangle. "

The triangle says, "I use it widely in my life." Such as triangular rulers, cranes, clothes hangers ... I am of great use. "

Quadrilateral said: "I have a wide range of uses, zoom ruler, movable iron gate ..."

Classification of triangles

Divide by angle

Judgment method 1:

1, acute triangle: all three internal angles of the triangle are less than 90 degrees.

2. Right triangle: One of the three internal angles of the triangle is equal to 90 degrees, which can be recorded as Rt△.

3. obtuse triangle: one of the three internal angles of the triangle is greater than 90 degrees.

Decision method 2:

1, acute triangle: the largest of the three internal angles of a triangle is less than 90 degrees.

2. Right triangle: The largest of the three internal angles of a triangle is equal to 90 degrees.

3. obtuse triangle: the largest of the three internal angles of a triangle is greater than 90 degrees and less than 180 degrees.

Among them, acute triangle and obtuse triangle are collectively called oblique triangle.