where I used that f is odd in the second step.
Suppose that f is an odd function which is differential for all x. Show that f' is even.
I know this is like an easy question but I'm totally forgetting how to right a formal proof for these because I haven't done them in so long...but I know like let f=2x+1 and then f' would be 2 which would be even..but I feel like I have the show the deriv as 2k not just 2 but I'm forgetting how to work through it lol thanks!