is that there's only one tangency portfolio

即只有一种切线投资组合
and that portfolio is called the tangency portfolio,

而那投资组合就叫做切线投资组合
where a line drawn from the risk--

画出来的话它是一条从风险
from the x-axis at the riskless rate

穿过x轴上的无风险利率点
is tangent to the efficient portfolio frontier.

与有效边界相切的直线
The tangency portfolio is the portfolio that one should hold.

切点投资组合就是我们应该持有的投资组合
The tangency portfolio gives rise to

切线投资组合引申出
what's called the mutual fund theorem in finance,

金融学里的的共同基金定理
which says that all investors need is a single mutual fund.

即所有的投资者只需要持有一份共同基金
Now, I haven't defined mutual fund yet.

现在我还没有给共同基金下一个定义
A mutual fund is an investment vehicle

共同基金是一种投资手段
that allows investors to hold a portfolio.

允许投资者持有一种投资组合