solution: ∵△ ABC ≌△ a ′ b ′ c ′

∴ AB = A1b, BC=BC1, ∠ABA1=∠CBC1

∵AA1∥BC.

2. On the plane, the sum of the external angles of a triangle is equal to 36 (external angle sum theorem).

3. On the plane, the outer angle of a triangle is equal to the sum of two inner angles that are not adjacent to it.

4. There are at least two acute angles among the three internal angles of a triangle.

5. At least one angle in the triangle is greater than or equal to 6 degrees, and at least one angle is less than or equal to 6 degrees.

6. In a right-angled triangle, if an angle is equal to 3 degrees, the right-angled side opposite to the 3-degree angle is half of the hypotenuse.