Question

[Nguyễn Quốc Khánh]

Tìm x biết :

8/1.5 + 8/5.9 + 8/9.13 +......+ 8/x(x+4) = 1/2

       Giúp mình với mình đang cần gấp!!!!!!!!!!!!!!!

Answer 1

[Không Tên]

    \(\frac{8}{1.5}+\frac{8}{5.9}+\frac{8}{9.13}+...+\frac{8}{x\left(x+4\right)}=\frac{1}{2}\)

\(\Leftrightarrow\)\(2\left(\frac{4}{1.5}+\frac{4}{5.9}+\frac{4}{9.13}+...+\frac{4}{x\left(x+4\right)}\right)=\frac{1}{2}\)

\(\Leftrightarrow\)\(1-\frac{1}{5}+\frac{1}{5}-\frac{1}{9}+\frac{1}{9}-\frac{1}{13}+...+\frac{1}{x}-\frac{1}{x+4}=\frac{1}{4}\)

\(\Leftrightarrow\)\(1-\frac{1}{x+4}=\frac{1}{2}\)

\(\Leftrightarrow\)\(\frac{x+4-1}{x+4}=\frac{1}{2}\)

\(\Leftrightarrow\)\(\frac{x+3}{x+4}=\frac{1}{2}\)

\(\Rightarrow\)\(2\left(x+3\right)=x+4\)

\(\Leftrightarrow\)\(2x+6=x+4\)

\(\Leftrightarrow\)\(x=-2\)

Vậy....

P/s: tham khảo mk ko chắc là đúng