来自《耶鲁大学开放课程：金融市场 第4集 第4集》

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  that's just the formula for the variance of the portfolio
  这是计算投资组合方差的公式
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  as a function of--Now, since they have to sum to 1,
  这是个函数...既然投资总额是1
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  I can write this as x12 σ12 + (1 - x1)2 σ22 + 2x1 (1 - x1) σ12
  也可以写成[公式如下]
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  and so that together traces out--
  由此我们得出...
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  I can choose any value of x1 I want,
  x1可以是任意值
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  it can be number from minus infinity to plus infinity.
  可以是从负无穷到正无穷之间的任何数值
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  That shows me then for any value of x1,
  运用这些公式 则给定任何数值的x1
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  I can compute what r is and what σ2 is
  我们都可以计算出r和σ2
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  and I can then describe the opportunities I have
  根据这些数据 我就能知道
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  from investing that depend on these.
  投资组合盈利的可能性有多大了
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  Now, one thing to do is to solve the equation for r and x1
  现在我们要解出r和x1的关系等式
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  and I can then recast the variance in terms of r ;
  然后改写第二条等式 用r来表示σ2