Pythagorean Theorem
You might know it as an equation that tells you the relationship between the sides of a right triangle.
Take any right triangle, if you know 2 of the sides, then you can find the 3nd side.
But we can also understand the relationship of the sides with pictures.
Check out this little video:
What is the Pythagorean Theorem?
The video shows it works for Post-It notes, but how about any triangle?
Check out some visual proofs in the wonderful video below.