Hi, I've already worked out the question until part (a) but I don't understand how they go from (a) to (b) right at the bottom of the image.(adsbygoogle = window.adsbygoogle || []).push({});

How did they change -s + i to s - i ?

[PLAIN]http://img64.imageshack.us/img64/9366/unledrze.png [Broken]

Thank you very much for your help.

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Need help Simplifying this Equation

**Physics Forums | Science Articles, Homework Help, Discussion**