Question

x+(x+1)+(x+2)+(x+3)+........+(x+30)=1240

Answer 1

\(x+\left(x+1\right)+\left(x+2\right)+\left(x+3\right)+...+\left(x+30\right)=1240\)

\(\Rightarrow x+x+x+...+x+1+2+3+...+30=1240\)

\(\Rightarrow31x+\left(1+2+3+...+30\right)=1240\)

Số hạng là:

\(\left(30-1\right):1+1=30\) (số hạng)

Tổng là:

\(\left(30+1\right)\cdot30:2=465\)

\(\Rightarrow31x+465=1240\\ \Rightarrow31x=1240-465\\ \Rightarrow31x=775\\ \Rightarrow x=25\)