The square root of pi, like pi itself, is an irrational number. It can be approximated to four decimal places by 1.7725. The factorial of 1/2 is equal to half of the square root of pi.
The concept of the mathematical constant pi dates back approximately 4,000 years to ancient Mesopotamia. Babylonian mathematicians found that the area of a circle could be calculated by taking the square of the radius and multiplying it by their closest approximation of pi, which was 3. The name pi came into use for this constant in the early 18th century, when British mathematician William Jones used the Greek character to denote the approximate ratio of a circle's circumference to its diameter.