Pythagoras was a Greek mathematician who lived in the 6th century BC and traveled extensively, learning about mathematics in Egypt. The Pythagorean theorem states that in a right triangle, the square of the hypotenuse is equal to the sum of the squares of the other two sides. This theorem can be used to solve real-world problems like calculating how far up a wall an 11m ladder will reach if its base is 4m from the wall.