Section 6: Functions 81
Factorials
The FACT function permits you to handle permutations and combinations with ease. To calculate the factorial of a positive integer in the X-register, execute the FACT function. For example, calculate the number of ways that six people can line up for a photograph.
P
 6 6
= 6! = 6 × 5 × 4 × 3 × 2 × 1
Keystrokes Display
6
 6 _

XEQ
 XEQ _ _

ALPHA FACT ALPHA
 720
The HP-41C places 9.999999999 × 1099 in the X-register and displays OUT OF RANGE for factorials of numbers greater than 69.
Square Roots
To calculate the square root of a number in the X-register, execute the   function. On the keyboard the function looks like this:   . And when you execute the function from the display or reassign the function, the name is SQRT
 Keyboard execution: Display execution: SQRT
Find the square root of 16 using   key on the keyboard:
Keystrokes Display
16
 6 _

 4

Now find the square root of the result using SQRT in the display:
Keystrokes Display

 4

XEQ
 XEQ _ _

ALPHA SQRT ALPHA
 2