Image: https://i.gyazo.com/ca53a195685574c842f8e8ed2ae30c69.jpg
My question is on the line below where it says "Along the path...": I have no idea where that x+z component came from. I'm sure it's from the parameterization of the straight segment, but I'm not sure what process you use to get that. Can someone show steps? Thank you.
Turns out @Bye_World was correct -- it was a typo. F was supposed to originally be given as < whatever, x+z, whatever> and was incorrectly given as < whatever, x, whatever>. At least it explains why I was so confused.