Question

x+(x+1) + (x+2) ......(x+30)=1240  ..

Answer 1

x+(x+1)+(x+2)+(x+3)+...+(x+30)=1240
x+x+1+x+2+...+x+30=1240
x+x+...+x+1+2+...+30=1240
x*31+(30+1)*30:2=1240
x*31+465=1240
x*31=1240-465
x*31=775
x=775:31
x=25

Answer 2

Số số hạng là :

\(\left(x+30\right)-x+1=31\) (số hạng)

Tổng trên là :

\(\left[\left(x+30\right)+x\right].31:2=\left(2x+30\right).31:2=1240\)

\(\Rightarrow\left(2x+30\right).31=2480\)

\(\Rightarrow2x+30=80\)

\(\Rightarrow2x=50\)

\(\Rightarrow x=25\)