Question

If n(n+1)/2=55 then find n​

Answer 1

$\dfrac{n(n+1)}{2}=55\\\\n(n+1)=110\\n^2+n-110=0\\n^2+11n-10n-110=0\\n(n+11)-10(n+11)=0\\(n-10)(n+11)=0\\n=10 \vee n=-11$

Answer 2

$\: \: \: \frac{n(n + 1)}{2} = 55 \\ or.n(n + 1) = 110 \\ or. {n}^{2} + n - 110 = 0 \\ or. {n}^{2} + (11 - 10)n - 110 = 0 \\ or. {n}^{2} + 11n - 10n - 110 = 0 \\ or.n(n + 11) - 10(n + 11) = 0 \\ or.(n + 11)(n - 10) = 0 \\ \\ \\ \: \: \: \: (n + 11) = 0 \\ > n = - 11 \\ \\ \\ \\ \\ \\ \\ \\ \\ (n - 10) = 0 \\ > n = 10$

Hope it will help you.....✌️✌️⤴️