Hi all!

I'm having some issues understanding where Casey derives the following form of the equation for reflecting the velocity vector about the wall vector: V - 2V·rr

I understand the alternative form intuitively: -V·rr + V·WW as it's easy for me to see how in this form the resultant vector is the combination of the 'r' and 'W' components of the velocity vector projected onto the r and W axes, respectively.

However I'm struggling to see how the other form is derived. Can anybody point me in the right direction?

Many thanks in advance!

- Jonny