\(1+3+5+....+x=36\)
Tổng của dãy là:
\(\frac{\left(x+1\right).\left[\left(x-1\right)\div2+1\right]}{2}=36\)
\(\Rightarrow\left(x+1\right).\left[\left(x-1\right)\div2+1\right]=36.2\)
\(\Rightarrow\left(x+1\right).\left[\left(x-1\right)\div2+1\right]=72\)
\(\Rightarrow\left(x+1\right).\left(x+1\right)=72.2\)
\(\Rightarrow\left(x+1\right).\left(x+1\right)=144\)
\(\Rightarrow\left(x+1\right)^2=144\)
\(\Rightarrow\left(x+1\right)^2=12^2\)
\(\Rightarrow x+1=12\Rightarrow x=11\)
Đúng 0
Bình luận (0)
