`\color{#a8b0b4}{⸻}\color{#91999d}{⸻}\color{#7a8286}{『}\color{#636b6f}{♡} \color{#4c5356}{E}\color{#434a4d}{m}\color{#3b4244}{p}\color{#2a3133}{t}\color{#21282a}{y}\color{#1a2123}{B}\color{#141a1c}{o} \color{#0f1516}{y} \color{#05090a}{♡}\color{#0a1011}{』}\color{#141a1c}{⸻}\color{#1a2123}{⸻}`

Chọn `2` điển trong `n-1` điểm có `{(n-1)(n-2)}/2` cách 

`{(n-1)(n-2)}/2=28`

`(n-1)(n-2)=28*2=56`

Do `7*8=56` nen `n-2=7`

Vậy `n=9`