Express in ${\displaystyle \underset{?}{\overset{?}{}}}$ notation

$15.1+4+9+16+$dotsdots.$\mathrm{.}100$

$16.1+3+5+7+9+$dots$\mathrm{.}101$

$17.x_1{y}_1+x_3{y}_3+x_5{y}_5+x_7{y}_7$

$18.n+n+n+$dots $(20$ times$)$

$19.x_1{y}_1+x_1{y}_2+$dots.$x_1{y}_{10}+x_2{y}_1+x_2{y}_2+x_2{y}_3+$dots..$x_2{y}_{10}$

$20.(1-2{)}^2+(1-3{)}^2+(1-4{)}^2+(1-n{)}^2$