Question

The sum of n tetms of AP.24,21,18,.......is 108.find n​

Answer 1

$sn = \frac{n}{2} (2a + (n - 1)d) \\ 108 = \frac{n}{2} (2(24) + (n - 1) - 3 \\ 108 \times 2 = n(48 - 3n + 3) \\ 216 = 48n - 3 {n}^{2} + 3n \\ 216 = 51n - 3 {n}^{2} \\ - 3 {n}^{2} + 5n - 216 \\ \ 3 {n}^{2} - 5n + 216 = 0$