1, the triangle is stable.

2. The sum of two sides of a triangle is greater than the third side, and the difference between the two sides is less than the third side.

3. The longer the side of a triangle, the bigger its corresponding angle.

4. For right-angled triangles: Let the hypotenuse be C and the right-angled sides be A and B respectively, then a 2+b 2 = c 2.

5. Let the three sides of a triangle be A, B and C respectively, and their corresponding angles are A, B and C respectively, then there are

Area s=√p(p-a)(p-b)(p-c)

(Helen formula) where p=(a+b+c)/2.

A/sina=b/sinb=c/sinc (sine theorem)

a^2=b^2+c^2-2bc*cosa

(Cosine theorem)

b^2=c^2+a^2-2ac*cosb

(Cosine theorem)

C 2 = A 2+B 2-2AB * COSC (Cosine Theorem)