习题：
　　Exercise：
　　Consider a portfolio with 40% invested in asset X and 60% invested in asset Y. The mean and variance of return on X are 0 and 25, respectively. The mean and variance of return on Y are 1 and 121, respectively. The correlation coefficient between X and Y is 0.3. What is the nearest value for portfolio volatility?
　　A. 9.51
　　B. 8.60
　　C. 13.38
　　D. 7.45
　　解析：
　　Answer: D
 
　　Explanation:
　　知识点：
　　Risk of Portfolios of Risky Assets
　　Variance and standard deviation (or volatility) of returns are common measures of investment risk. The variance of returns for a portfolio of two risky securities is not a simple weighted average of the variances of the two securities. It depends on how the returns on the securities move together, which is measured by the covariance of the returns on the two securities.
 
　　Since  , variance can also be written as:
 
　　Standard deviation or volatility can be written as:
 

 
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