\(\dfrac{1}{\left(x+1\right)\left(x+2\right)}+\dfrac{1}{\left(x+2\right)\left(x+3\right)}+....+\dfrac{1}{\left(x+2020\right)\left(x+2021\right)=1}\)
\(\Leftrightarrow\dfrac{1}{x+1}-\dfrac{1}{x+2}+\dfrac{1}{x+2}-\dfrac{1}{x+3}+...+\dfrac{1}{x+2020}-\dfrac{1}{x+2021}=1\)
\(\Leftrightarrow\dfrac{1}{x+1}-\dfrac{1}{x+2021}=1\)
\(\Leftrightarrow\dfrac{\left(x+2021\right)-\left(x+1\right)}{\left(x+1\right)\left(x+2021\right)}=1\)
\(\Leftrightarrow\dfrac{x+2021-x-1}{\left(x+1\right)\left(x+2021\right)}=1\)
\(\Leftrightarrow\dfrac{2020}{\left(x+1\right)\left(x+2021\right)}=1\)
\(\Leftrightarrow\left(x+1\right)\left(x+2021\right)=2020\)
\(\Leftrightarrow x^2+2021x+x+2021=2020\)
\(\Leftrightarrow x^2+2022x=-1\)
\(\Leftrightarrow x\left(x+2022\right)=-1\)
Đến đây bạn chia trường hợp để giaỉ ra nghiệm nguyên nhé
