Question

Tìm x, biết rằng: (x + 2) + (x + 4) + … + (x + 20) = 998000

Answer 1

\(\left(x+2\right)+\left(x+4\right)+...+\left(x+20\right)=998000\)

\(\Rightarrow\left(x+x+....+x\right)+\left(2+4+6+...+20\right)=998000\)

Số số hạng là: (20-2):2+1 = 10 (số)

Tổng dãy 2+4+6+...+20 là: (20+2) x 10 : 2 = 110

\(\Rightarrow10x+110=998000\)

\(\Rightarrow x=99789\)

Answer 2

(x + 2) + (x + 4) + ... + (x + 20) = 998000

x + 2 + x + 4 + ... + x + 20 = 998000

10x + (2 + 4 + ...  + 20) = 998000

 \(10x+\frac{\left(20+2\right)\left[\left(20-2\right):2+1\right]}{2}=998000\)

10x + 110 = 998000

10x = 997890

x = 99789