The root number 3 is the square root of 3. The root number 3 is a mathematical symbol, representing the square root of 3, an irrational number, an infinite cycle decimal, 1.732, which is an important constant in mathematics. The geometric meaning of the root number 3 is to represent the trisection of a circle with a length equal to the radius. The root sign means the square of a number, taking a certain number of roots. The root number 3 plays an important role in Pythagorean Theorem, because it is closely related to the right triangle in Pythagorean Theorem. When the lengths of the two right sides of a right triangle are 1 and 2 respectively, the length of the hypotenuse is root number 3.