# How to deal with this equation involving a plus/minus term

1. Mar 30, 2015

Hey guys,

So I'm reading a textbook which has the following equation:

$\dot{X}^{-}\pm X^{-\prime}=\dfrac{1}{4\alpha' p^{+}}\left( \dot{X}^{I}\pm X^{I\prime} \right)^{2}$.

Please note that the $+,-,I$ are indices. Then the author says:

$\dot{X}^{-}= \dfrac{1}{4\alpha' p^{+}}\left( \dot{X}^{I}\dot{X}_{I}+X^{I\prime}X_{I}^{'} \right)$

where the repeated up and down index is using Einstein's summation convention.

I have no idea how you can get this equation from the first one...can someone explain please?

Thanks...

2. Mar 31, 2015

### robphy

What happens if you write the first as two equations?
One with +, the other with -. Expand their right hand sides. Then add...