Error amplification

Two bank stocks have volatilities 18% and 20% and correlation 0.9; risk-free 6.5%. (a) Compute $\Sigma^{-1}$ and the tangency weights for expected returns (13%, 14%), then (13%, 15%). (b) Express the change in unnormalised weights as $\delta\,\Sigma^{-1}e_2$ and explain the amplification factor $1/(1-\rho^2)$. (c) The standard error of a 5-year sample mean for a 20%-volatility stock is about how large? Comment.