Question

find n if
1+2+3+...
+n = 210​

Answer 1

Answer:

204

Step-by-step explanation:

1+2+3+n=210

6+n=210

n=210-6

n=204

Answer 2

Answer:

n=20

Step-by-step explanation:

$1 + 2 + 3 + ............ + n = 210 \\ formula = > \\ 1 + 2 + 3 + ............ + n = \frac{n(n + 1)}{2} \\ \frac{n(n + 1)}{2} = 210 \\ n(n + 1) = 420 \\ {n}^{2} + n - 420 = 0 \\ {n}^{2} + 21n - 20n - 420 = 0 \\ n(n + 21) - 20(n + 21) = 0 \\ (n + 21)(n - 20) = 0 \\ n + 21 = 0 \: \: and \: \: n - 20 = 0 \\ n = - 21 \: \: (not \: possibal) \: \: and \: \: \: n = 20 \\ n = 20 \: \: \: (ans)$