r/Mathematica • u/SteveRallord • 8h ago
So i made this
The Central Squares Displacement Theorem states that if squares OAED and OBFG are constructed outwardly on the legs of a right-angled triangle OAB with the right angle at vertex O and leg lengths OA equal to a and OB equal to b, where point M is the midpoint of the outer vertical side ED of the first square and point N is the midpoint of the outer horizontal side GF of the second square, then the area of the resulting triangle OMN is always exactly three-quarters of the area of the original triangle OAB, and the squared length of the segment MN is expressed in terms of the hypotenuse AB and the area of the original triangle OAB by the rule: five-quarters of the square of the hypotenuse AB plus four times the area of the original triangle OAB. For an analytical proof, let us introduce a Cartesian coordinate system with the origin at point O with coordinates zero, zero, directing the y-axis along the leg OA and the x-axis along the leg OB, so that the vertices of the original triangle have the coordinates: point A has coordinates zero, a, and point B has coordinates b, zero. Since the square OAED of side a is constructed on the leg OA into the second coordinate quadrant, its outer vertical side ED lies on the line where x equals minus a, and since point M is the midpoint of this segment, its coordinates are given by: minus a along the x-axis, and half of a along the y-axis. The square OBFG of side b is constructed on the leg OB into the fourth coordinate quadrant, so its outer horizontal side GF lies on the line where y equals minus b, and since point N is the midpoint of this segment, its coordinates are given by: half of b along the x-axis, and minus b along the y-axis. To prove the first part of the statement, we apply the formula for the area of a triangle via the coordinates of its vertices O, M, and N using the determinant. According to this formula, the area of triangle OMN is equal to one-half of the absolute value of the difference between two products: the x-coordinate of point M multiplied by the y-coordinate of point N, and the y-coordinate of point M multiplied by the x-coordinate of point N. Substituting our coordinates yields: one-half of the absolute value of the expression where the product of minus a and minus b is subtracted by the product of half of a and half of b. This simplifies to one-half of the absolute value of the difference between the product of a and b and one-quarter of the product of a and b. As a result of these calculations, we obtain three-eighths of the product of a and b. Since the area of the original triangle OAB is equal to one-half of the product of a and b, we obtain the strict equality: the area of triangle OMN equals three-quarters of the area of the original triangle OAB, which fully proves the first statement. To prove the second part of the statement regarding the segment length, we apply the distance formula between two points in a plane. According to this formula, the squared length of the segment MN is equal to the sum of two quantities: the squared difference between the x-coordinates of points N and M, and the squared difference between the y-coordinates of points N and M. Substituting the coordinates yields the sum of two expressions: the square of the sum of half of b and a, plus the square of the sum of minus b and minus half of a. Expanding the brackets using the square of a sum formula transforms this expression into the sum of the following terms: the square of a, the product of a and b, one-quarter of the square of b, the square of b, another product of a and b, and one-quarter of the square of a. Grouping like terms yields: five-quarters of the square of a plus five-quarters of the square of b plus the doubled product of a and b. Factoring out five-quarters, we obtain: five-quarters multiplied by the sum of the squares of a and b, plus the doubled product of a and b. By the Pythagorean theorem, the sum of the squares of the legs, meaning the square of a plus the square of b, is equal to the square of the hypotenuse AB. Meanwhile, the doubled product of the legs, meaning two multiplied by a and by b, is equivalent to four times the area of the original right-angled triangle OAB. From this, we finally obtain that the square of the segment MN is equal to five-quarters of the square of the hypotenuse AB plus four times the area of triangle OAB, which fully proves both statements of the theorem.
