The Standard Deviation for a Random Variable with Probability Density $f$

Multiple Choice

The standard deviation for a random variable with probability density function $f$ and mean $\mu$ is defined $\sigma = \left[ \int _ { - \infty } ^ { \infty } ( x - \mu ) ^ { 2 } f ( x ) d x \right] ^ { 1 / 2 } .$ Find the standard deviation for an exponential density function with mean 10 .

A) 10
B) 2
C) 5
D) $3.3$
E) $2.5$