Chapter 5 - Number Theory and the Real Number System - 5.7 Arithmetic and Geometric Sequences - Exercise Set 5.7 - Page 329: 56

The first six terms are: $20, -80, 320, -1280, 5120, \text{ and } -20480$.

In a geometric sequence, the next term can be found by multiplying the common ratio $r$ to the previous term. Thus, with $a_1=20$ and $r=-4$, the first six terms are: $a_1=20\\ a_2=20(-4)=-80\\ a_3=-80(-4)=320\\ a_4=320(-4)=-1280\\ a_5=-1280(-4)=5120\\ a_6=5120(-4)=-20480$