51 + 52 + 53 + ... + 100 = ?
2525
2975
3225
3775
Explanation:
Sn = $\left(1 + 2 + 3 + ... + 50 + 51 + 52 + ... + 100\right)$ - $\left(1 + 2 + 3 + ... + 50\right)$
=$ \dfrac{100}{2} $ x (1 + 100) -$ \dfrac{50}{2} $x (1 + 50)
= $\left(50 \times 101\right)$ - $\left(25 \times 51\right)$
= 5050 - 1275
= 3775.