In: Math
Prove the following problems using the complex plane model of Euclidean geometry, in the spirit of Erlangen Program: 1. Prove that the diagonals of a parallelogram bisect each other. 2. Prove that the sum of the squares of the diagonals of a parallelogram is equal to the sum of the squares of all the sides of the parallelogram. 3. Prove the Cosine Law for triangles: In a triangle with the sides a, b, and c, the square of the side opposite C = is expressed as c2 = a2 + b2 - 2 b a cos . 4. Prove the theorem: The bisector of an angle of a triangle divides the opposite side into two segments which are proportional to the sides that include the angle.