Congruent triangles, the first volume of Mathematics in Grade Two.

Prove:

Because BD=CE,

So BD-CD=CE-CD

So BC=DE

In triangle ABC and triangle ADE

AB=AE

{AC=AD

BC=DE

So triangle ABC is equal to triangle ADE (SSS)

Divide by angle

Judgment method:

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.