# finding the standard deviation of a radom variable??

• Apr 29th 2009, 10:50 AM
arslan
finding the standard deviation of a radom variable??
can anybody solve this.
• Apr 29th 2009, 10:55 AM
Moo
Quote:

Originally Posted by arslan
can anybody solve this.

Compute $\displaystyle \mathbb{E}(X)=\sum_{i=1}^3 x_i \mathbb{P}(X=x_i)=0.2 \times 10+0.3\times 20+0.5\times 30$

Complete the table with the values of x^2
Then compute $\displaystyle \mathbb{E}(X^2)=\sum_{i=1}^3 x_i^2 \mathbb{P}(X=x_i)$

The variance is $\displaystyle \mathbb{E}(X^2)-[\mathbb{E}(X)]^2$

That's all folks
• Apr 29th 2009, 11:08 AM
arslan
Quote:

Originally Posted by Moo
Compute $\displaystyle \mathbb{E}(X)=\sum_{i=1}^3 x_i \mathbb{P}(X=x_i)=0.2 \times 10+0.3\times 20+0.5\times 30$

Complete the table with the values of x^2
Then compute $\displaystyle \mathbb{E}(X^2)=\sum_{i=1}^3 x_i^2 \mathbb{P}(X=x_i)$

The variance is $\displaystyle \mathbb{E}(X^2)-[\mathbb{E}(X)]^2$

That's all folks

i dont quite get where do i go once i do 10x0.2+20x0.3+30x0.5 = 23
• Apr 29th 2009, 04:40 PM
mr fantastic
Quote:

Originally Posted by arslan
i dont quite get where do i go once i do 10x0.2+20x0.3+30x0.5 = 23

You go and do the second and third things that you were told in post #2.