Chapter 11 - Further Topics in Algebra - 11.2 Arithmetic Sequences and Series - 11.2 Exercises - Page 1024: 31

$a_n=s+2pn-2p$ $a_8=s+14p$

$a_4=s+6p$ $d=2p$ $a_n=a_1+(n-1)d$ $a_4=a_1+(4-1)d$ $s+6p=a_1+3*(2p)$ $s+6p=a_1+6p$ $a_1=s$ $a_n=a_1+(n-1)d$ $a_n=s+(n-1)*(2p)$ $\boxed{a_n=s+2pn-2p}$ $a_8=s+2p*8-2p$ $a_8=s+16p-2p$ $\boxed{a_8=s+14p}$