1, sine theorem: For △ABC, three sides are A, B and C respectively, then there is: a/sinA=b/sinB=c/sinC=2R (R is the radius of its circumscribed circle).

2. Cosine theorem: For △ABC, the three sides are A, B and C respectively, then there is: A * 2 = B * 2+C * 2-2 BC COSAB * 2 = A * 2+C * 2-2 AC COSB C * 2 = B * 2+A * 2-2 BACOSB.

3. Area formula:

S =1/2absinc =1/2acsinb =1/2bcsina = ABC/(4r) (A, B and C are three sides of a triangle respectively, A is the included angle between B and C, and others are similar; R is the radius of its circumscribed circle)

Helen formula: Let three sides of a triangle be A, B, C, p= 1/2(a+b+c), then the area is S = √ [P (P-A) (P-B) (P-C)].

4. Projective theorem

5. Intersecting chord theorem

6. For Rt△, hypotenuse C, right angles A and B, and the radius of inscribed circle R, there are: r=(a+b+c)/2.

7. The intersection (center of gravity) of the center line of the triangle is divided into two sections, and the length ratio of these two sections is 2: 1.

8. The list is endless. The above are common, commonly used and often overlooked.