kinda confused on the change of variable stuff (1 Viewer)

[Run hard@thehsc]

Can someone explain to me how this change of variable works for the reflection property: how can we assume that when u = a - x, then x = u?
Thanks!!!

 

[cossine]

Can someone explain to me how this change of variable works for the reflection property: how can we assume that when u = a - x, then x = u?
Thanks!!!

What theorem are you referring? Can you type it in Latex.

 

[d1zzyohs]

I believe he might be referring to this.
It's basically just a u-sub.