Question

tìm x biết : 1+ 3+ 5+ .....+ x =1600 ( x là số tự nhiên lẻ )

Answer 1

\(\text{1+ 3+ 5+ .....+ x =1600 }\)

\(\left\{\frac{\left(n-1\right)}{2+1}\right\}^2=1600\)

\(\left\{\frac{\left(n-1\right)}{2+1}\right\}^2=40^2\)

\(\left\{\frac{\left(n-1\right)}{2+1}\right\}=40\)

\(\left\{\frac{\left(n-1\right)}{2}\right\}=40-1\)

\(\left\{\frac{\left(n-1\right)}{2}\right\}=39\)

\(n-1=39.2\)

\(n-1=78\)

\(n=78+1\)

\(n=79\)