Factorial

Well 999! is the equivalent to 999*998*997*996*995*... until 1. So it is technically not infinity but multiplying all those number is very difficult for any calculator to do, and it takes to long to process so as far as we are concerned it is infinite.

How does 999! = infinity

999!=4.02387260077093773543702433923 x 10^2,564

999! is too big for the calculator to show.