Question

\(4+6+8+...+\)x=\(990\)

Tìm x 

Answer 1

Áp dụng công thức tính tổng dãy số , ta có :

\(\frac{\left[\left(x-4\right):2+1\right].\left(x+4\right)}{2}=990\)

\(\left(\frac{x-4}{2}+1\right)\left(x+4\right)=1980\)

\(\frac{x-2}{2}.\left(x+4\right)=1980\)

\(\left(x-2\right)\left(x+4\right)=3960\)

\(x^2+4x-2x-8=3960\)

\(x^2+2x=3968\)

\(x\left(x+2\right)=3968=62.64\)

=> x = 62