In the diagram, the vertices are (0,0), (a,0), (b,c), (a+b,c). All edges measure a. The diagonals are T1 and T2.
As you see, there's a right triangle with edges a, b and c. From pythagorean theorem, we find .
If two lines are perpendicular, the product of their gradients is -1.
Product of them
Remember we found before. Then,