p5

To calculate $P(X=a,b,c,d,\dots )$ when $X\sim \mathrm{Po}(\lambda )$

$$P(X=a,b,c,d,\dots )=e^{-\lambda }\left(\frac{{\lambda }^a}{a!}+\frac{{\lambda }^b}{b!}+\frac{{\lambda }^c}{c!}+\frac{{\lambda }^d}{d!}+\dots \right)$$

Example: Finding $P(X=0,1,2)$ when $\lambda =4.5$

1. Type the following and ensure cursor it at writing level afterwards (not in exponent)

$$e^{-4.5}$$

2. Press SHIFT + ${\log }_{x}Y$ (button below ON with a yellow summation sign above it)
3. Type the following:

$$\frac{4.5^X}{X!}$$

4. Press arrow up 2 times
5. Type the upper bound in this case it is 2
6. Press arrow down
7. Type the lower bound, in this case it is 0
8. It will calculate the appropriate value