\(\frac{1}{\left(x+4\right)\left(x+5\right)}+\frac{1}{\left(x+5\right)\left(x+6\right)}+\frac{1}{\left(x+6\right)\left(x+7\right)}\)
\(\frac{1}{x+4}-\frac{1}{x+5}+\frac{1}{x+5}-\frac{1}{x+6}+\frac{1}{x+6}-\frac{1}{x+7}=\frac{1}{18}\)
\(\frac{1}{x+4}-\frac{1}{x+7}=\frac{1}{18}\)
\(\frac{1}{x^2+9x+20}+\frac{1}{x^2+11x+30}+\frac{1}{x^2+13x+42}=\frac{1}{18}\)
\(\Rightarrow\frac{1}{\left(x+4\right)\left(x+5\right)}+\frac{1}{\left(x+5\right)\left(x+6\right)}+\frac{1}{\left(x+6\right)\left(x+7\right)}=\frac{1}{18}\)
\(\Rightarrow\frac{1}{x+4}-\frac{1}{x+5}+\frac{1}{\left(x+5\right)}-\frac{1}{x+6}+\frac{1}{x+6}-\frac{1}{x+7}=\frac{1}{18}\)
\(\Rightarrow\frac{1}{x+4}-\frac{1}{x+7}=\frac{1}{18}\)
\(\Rightarrow\frac{3}{\left(x+4\right)\left(x+7\right)}=\frac{1}{18}\)
\(\Rightarrow x^2+11x+28=54\)
\(\Rightarrow x^2+11x+\frac{121}{4}-\frac{9}{4}=54\)
\(\Rightarrow\left(x+\frac{11}{2}\right)^2=\frac{225}{4}\)
\(\Leftrightarrow\orbr{\begin{cases}x+\frac{11}{2}=\sqrt{\frac{225}{4}}\\x+\frac{11}{2}=-\sqrt{\frac{225}{4}}\end{cases}}\)
\(\Leftrightarrow\orbr{\begin{cases}x+\frac{11}{2}=\frac{25}{2}\\x+\frac{11}{2}=-\frac{25}{2}\end{cases}}\Leftrightarrow\orbr{\begin{cases}x=7\\x=-18\end{cases}}\)
@Cool boy @ vào sửa lại lỗi sai.
\(\sqrt{\frac{225}{4}}=\frac{15}{2}\)
Đáp án sai rồi nhé Cool.
Oh@@, sai vô lí:
\(\orbr{\begin{cases}x+\frac{11}{2}=\frac{15}{2}\\x+\frac{11}{2}=-\frac{15}{2}\end{cases}}\Leftrightarrow\orbr{\begin{cases}x=2\\x=-13\end{cases}}\)
