What does the floating point number $(1.2)$ down the bracket mean?

Question

What does the 1.2 on the image mean?

Thanks in advance.

Answer 1

It seems that they are indices 1 and 2 separated by a comma. Thus you have two different expressions. The $\pm$ distinguish between the two expressions:

$$\left( \frac{R_p}{r_{1}} \right)_{\color{red}1}=\frac{-C\color{red}+\sqrt{C^2-4(1-C)(-\sin^2(\gamma_1))}}{2(1-C)}$$

$$\left( \frac{R_p}{r_{1}} \right)_{\color{red}2}=\frac{-C\color{red}-\sqrt{C^2-4(1-C)(-\sin^2(\gamma_1))}}{2(1-C)}$$

It is an application of the $\text{quadratic formula}$. See here.