# [SOLVED] Probability (probability mass function,pmf)

• December 14th 2009, 11:21 PM
nameck
[SOLVED] Probability (probability mass function,pmf)
If X has the probability mass function f(x) = k / x! (x=0,1,2,...),
what is the value of k?
• December 15th 2009, 03:11 AM
HallsofIvy
Quote:

Originally Posted by nameck
If X has the probability mass function f(x) = k / x! (x=0,1,2,...),
what is the value of k?

You must have $\sum_{x=0}^\infty \frac{k}{x!}= k\sum_{x=0}^\infty \frac{1}{x!}= 1$.
• December 15th 2009, 03:18 AM
nameck
how to cancel the factorial(!) sign?
• December 15th 2009, 06:30 AM
Plato
Quote:

Originally Posted by nameck
how to cancel the factorial(!) sign?

At this level you are expected to know that $\sum\limits_{x = 0}^\infty {\frac{1}{{x!}}} = e$.