It's like a fourth asset but we're using a special feature of this asset: that it has no risk.

如同第四种资产,我们将利用这个资产的特性：,即无风险特性。

耶鲁公开课 - 金融市场课程节选

Tangent means that it has the same slope, it just touches the efficient portfolio frontier for risky assets at one point, and the slope of the efficient portfolio frontier, including the riskless asset, is a straight line that goes through the tangency point, here.

相切意味着斜率相同，它与包含风险资产的有效边界,交与一点，而包含无风险资产的，有效边界，则是一条过切点的直线，切点在这里。

耶鲁公开课 - 金融市场课程节选