Please give the proof of the relation theorem between the diagonal and the side of a parallelogram (the sum of the squares of the two diagonals of a parallelogram is equal to the sum of the squares of its four sides)

Using cosine theorem

RELATED INFORMATIONS

1. Proof: the square sum of two diagonals of a parallelogram is equal to the square sum of four sides
2. Verification: the sum of squares of two diagonals of a parallelogram is equal to the sum of squares of four sides
3. Proof: the square sum of two diagonals of a parallelogram is equal to the square sum of four sides
4. Prove that the sum of squares of four sides of a parallelogram is equal to the sum of squares of diagonals
5. If the side length of a diamond is 2, then the square sum of the two diagonals of the diamond is 2______ ．
6. If the side length of a diamond is 2, then the square sum of the two diagonals of the diamond is 2______ ．
7. If the side length of a diamond is 2, then the square sum of the two diagonals of the diamond is 2______ ．
8. If the side length of a diamond is 2, then the square sum of the two diagonals of the diamond is 2______ ．
9. How many times is the sum of the squares of the two diagonals of the diamond equal to the square of one side?
10. Proof: the area of the diamond is equal to half of the product of two diagonals
11. The proof that the sum of squares of diagonals of parallelogram is equal to the sum of squares of four sides RT I hope to be more detailed. If some trigonometric functions are involved, I also hope to explain them
12. Prove with vector method: the bisection sum of two diagonals of parallelogram is equal to twice of the square sum of two adjacent sides If the title, the best process Thank you! On the first floor Is the vector method that simple? ==|||I feel the process is so short No Of course, I think it's better to be short But the teacher won't let me But forget it Thank you for your quick answer ==|||I hate winter vacation homework Ah, ah
13. Prove with complex number: the sum of squares of parallelogram diagonal is equal to the sum of squares of four sides
14. It is proved that the sum of squares of the diagonals of a parallelogram is equal to the sum of squares of its sides
15. The average of known samples 9.8, 9.9, x, 10, 10.2 is 10, and the variance is calculated
16. Variance formula: the average of samples 9.8, 9.9, x, 10, 10.2 is 10, and the variance is calculated
17. For a group of data x1, X2, X3, X4 composed of positive integers, the average and median are 2, and the standard deviation is 1, then this group of data is___ (from small to large)
18. For a group of data x1, X2, X3, X4 composed of positive integers, the average and median are 2, and the standard deviation is 1, then this group of data is___ (from small to large)
19. The sum of squares of 50 data is 800, and the average is 3. Find the standard deviation of the 50 data
20. We know that the average of the sum of squares of a group of data is 228, and the average is 15. We can find the variance of this group of data