The three sides of a triangle are A, B and C. Let p=(a+b+c)/2.

Then the area of the triangle s=(p(p-a)(p-b)(p-c)).

Parallelogram theorem:

The sum of squares of two diagonals of a parallelogram is equal to the sum of squares of its four diagonals.

Angular bisector theorem;

In triangle ABC, if the bisector of angle A passes through BC and D, there is an equation.

AB/AC=BD/CD

Formula in the circle:

Intersecting chord theorem, secant theorem, secant theorem.