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.