Pi

One of the more important mathematical constants, Pi is equal to the length of a circle's circumference, divided by its diameter. 

No one knows the exact value of Pi. That's because it belongs to a class of numbers known as "irrational", numbers that cannot be expressed by a finite number of digits. A recently-computed approximation of Pi had over 1 trillion digits, still leaving an infinite number of digits to be determined.

For most purposes, a value of Pi accurate to 5-10 digits is sufficient. Karen's Calculator program can insert an approximate value of Pi into a calculation. That program lets you choose the precision of the value, ranging from one digit to the right of the decimal point, to as many as 10,000 digits to the left of the decimal point. 

The value of Pi, accurate to 50 digits, is: 3.14159265358979323846264338327950288419716939937511