115. $a_n=2^{4-n}=16{\left({\displaystyle \frac{1}{2}}\right)}^n.b_n={\displaystyle \frac{4}{2^{n-2}}}=16{\left({\displaystyle \frac{1}{2}}\right)}^n.$ These two sequences are the same.

117. $b_{1001}$ is larger

119. $n=15,k=16$

121. 2, 5, and 8; or 26, 5, and $-16$