Question

Tìm n biết: 1+2+3+4+.....+n=666

Answer 1

 1 + 2 + 3 + ... + n = 666

=> ( 1 + n ) . n : 2 = 666

=> n ( n + 1 ) = 1332

Vì 36 . 37 = 1332 .

=> n = 36 .

Vậy n = 36 .

Answer 2

\(1+2+3+4+......+n=666\)

\(\Rightarrow\dfrac{n\times\left(n+1\right)}{2}=666\)

\(\Rightarrow n\times\left(n+1\right)=1332\)

\(\Rightarrow n=16\)