Question

Tìm x biết (x+1)+(x+6)+(x+11)+...+(x+96)=990

Answer 1

20x +(1+6+11+.....+96)=990

20x + 970=990

20x=990-997

20x=20

x=1

 

Answer 2

(x+1)+(x+6)+...+(x+96)=990

\(\Leftrightarrow\)20x+(1+6+11+...+96) = 990

\(\Leftrightarrow\)20x + \(\frac{\left(1+96\right)\times\left[\left(96-1\right):5+1\right]}{2}\)= 990

\(\Leftrightarrow\)20x+970=990

\(\Leftrightarrow\)20x=990-970=20

\(\Leftrightarrow\)x=1

Vậy x= 1