Question

how did they go to step 2
http://prntscr.com/3hvgfc

Answer 1

just change the index of the summation.

Answer 2

It should just be rewriting the index, but let me write it out myself first
moment

Answer 3

yes they changed the binomial coefficient also, that part is confusing

Answer 4

substitute j with k-1 and tweak the summation limits accordingly. the new limits will be k from 1 to m+1. Restrict it to m, and you will get b^(m+1) as the left out term.

Answer 5

j = k-1

j : 0->m
k : 1->m+1

Answer 6

yup

Answer 7

oh and they pushed out the last term b^(m+1) out of summation, sweet xD

Answer 8

thank you :)

Answer 9

continuing on the next board...