MCQ
The remainder on dividing $1+3+3^{2}+3^{3}+\ldots+3^{2021}$ by $50$ is
- A$5$
- ✓$4$
- C$2$
- D$6$
$=\frac{(10-1)^{1011}-1}{2}$
$=\frac{100 \lambda+10110-1-1}{2}$
$=50 \lambda+\frac{10108}{2}$
$=50 \lambda+5054$
$=50 \lambda+50 \times 101+4$
$\operatorname{Rem}(50)=4 .$
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