Question

Tìm số tự nhiên x biết

9+10+11+12+...+x=1239

cần trả lời gấp

Answer 1

\(9+10+11+...+x=1239\)

\(\Leftrightarrow\frac{\left(9+x\right)\left(x-9+1\right)}{2}=1239\)

\(\Leftrightarrow\left(x+9\right)\left(x-8\right)=1239\times2\)

\(\Leftrightarrow x^2+x-72=2478\)

\(\Leftrightarrow\left(x+1\right)x=2478+72\)

\(\Leftrightarrow\left(x+1\right)x=2550=50\cdot51\)

\(\Rightarrow x=50\)