important points in 7.5.4:

- - understanding discrete vs. continuous
  - for a continuous random variable, probabilities of an interval of values are represented by areas (that are sectioned off by vertical slices).
  - for a continuous random variable the probability that x=( any specific value) = 0. for a continuous variable there are an infinite number of possible values the variable can have. even if you think of each one as equally likely (as in a uniform distribution), the probability of any one particular value is 1/(infinity) which is 0.
  - " expected value " is a term that means the mean (average)
  - the mean (“expected value”) is the "balance point" of the area