`190 - [ n (n-1) :2]+1=170`
`=> 190 - [n(n-1) :2]= 169`
`=> n(n-1):2 = 190 -169`
`=> n(n-1):2 = 21`
`=> n(n-1)= 21.2`
`=> n(n-1)= 42`
`=> n^2 - n =42`
`=>n^2 - n -42=0`
`=> n^2 - 7n + 6n-42=0`
`=> n(n-7) + 6(n-7)=0`
`=>(n-7)(n+6)=0`
`=>` \(\left[ \begin{array}{l}n-7=0\\n+6=0\end{array} \right.\)
`=>` \(\left[ \begin{array}{l}n=7\\n=-6\end{array} \right.\)
Vậy `n=7` hoặc `n= -6`
