hw5 clarification

[bbolker]

• in Q4, the "mean of the underlying distribution" (2) is not expected to be the same as the mean of the sampled values; the mean of the log-normal is exp(mu+sigma^2/2), where mu is the mean of the underlying distribution

[bbolker]

• in the updated version, Q3 (Jensen's inequality) doesn't really exist
• when solving Q2, go ahead and ignore the covariance term