Ta có $U_{n}=3^n+2046$
Suy ra $B=U_{25}+U_{26}+U_{27}+...+U_{41}$
$B=3^{25}+3^{26}+3^{27}+...+3^{41}+17.2046$
$B=\left(3+3^2+3^3+...+3^{41}\right)-\left(3+3^2+3^3+...+3^{24}\right)+17.2046$
$B=\dfrac{3^{42}-3}{2}-\dfrac{3^{25}-3}{2}+17.2046$
$B=54709494565756179603-423644304720+34782$
Vậy $B=54709494142111909665$