Can you use this formula to find $S_{20}$, $S_{50}$ or $S_{1000}$?                                                                                                       [Page No. 185]

Yes. Using $S_n = \frac{n(n+1)}{2}$ for the sum of the first $n$ natural numbers:

$$S_{20} = \frac{20 \times 21}{2} = \frac{420}{2} = 210$$

$$S_{50} = \frac{50 \times 51}{2} = \frac{2550}{2} = 1275$$

$$S_{1000} = \frac{1000 \times 1001}{2} = \frac{1001000}{2} = 500500$$