Question

[Hoa bà chằn]

1+2+3+4...+n=9999

Answer 1

[Nguyễn Thế Tùng]

1+2+3+4+...+n=9999

=) \(\frac{\text{(1+n)n}}{2}\)=9999

=) (1+n)n=9999*2

=) n+n^2=19998

tự lm típ nhé