$\begin{array}{l}3)\\a)\,\,\text{- Rút gọn phân số ta được :}\\\dfrac68=\dfrac34\\\dfrac{-12}{15}=\dfrac{-4}5\\\dfrac{18}{24}=\dfrac34\\\dfrac{-24}{30}=\dfrac{-4}5\\\dfrac{36}{48}=\dfrac34\\\dfrac{40}{-50}=\dfrac{-4}5\\\text{- Vậy các nhóm phân số bằng nhau là :}\\\dfrac68=\dfrac{18}{24}=\dfrac{36}{48}\\\dfrac{-12}{15}=\dfrac{-24}{30}=\dfrac{40}{-50}\\\,\\b)\,\,\text{- Rút gọn phân số ta được :}\\\dfrac{-6}{-10}=\dfrac35\\\dfrac{10}{12}=\dfrac56\\\dfrac{-25}{35}=\dfrac{-5}7\\\dfrac{15}{-21}=\dfrac{-5}7\\\dfrac{30}{36}=\dfrac56\\\dfrac{36}{60}=\dfrac35\\\text{- Vậy các nhóm phân số bằng nhau là :}\\\dfrac{-6}{-10}=\dfrac{36}{60}\\\dfrac{10}{12}=\dfrac{30}{36}\\\dfrac{-25}{35}=\dfrac{15}{-21}\\\,\\6)\\\text{- Đặt $d \in ƯC(n-1,n-2)\,\,(d\in\mathbb{Z})$}\\\to \begin{cases} n-1\,\,\vdots\,\,d\\n-2\,\,\vdots\,\,d\end{cases}\\\to (n-1)-(n-2)\,\,\vdots\,\,d\\\to 1\,\,\vdots\,\,d\\\to d=\pm1\\\to\text{$\dfrac{n-1}{n-2}$ tối giản với mọi $n\in\mathbb{Z}, n\neq2$} \end{array}$
