Question

:) Sn = 100, a = 5, d = 5, n =?​

Answer 1

$sn = \frac{n}{2} ({ 2a + (n - 1)d}) \\ = > 100 = \frac{n}{2} (2 \times 5 + (n - 1) \times 5) \\ = > 100 = \frac{n}{2} (10 + 5n - 5) \\ = > 100 = \frac{n}{2} (5 + 5n) \\ = > 200 = 5n + 5n {}^{2} \\ = > 5n {}^{2} + 5n - 200 = 0 \\ = > n {}^{2} + n - 40 = 0$