It's my own theory. I also have my own formulas.
EXAMPLE
AH Isoceles.
Take square of any 2 same numbers. Add them and find the under root of the answer.
Next take the same number and divide it by 0.707. It goes like:
72+72= 49+49= /98 = 9.899 (1)
Now my formula:
7/0.707= 9.900 (2)
It's a little different. Just round off the first value. The next time I'll avoid pythagoras theorem in isoceles.