Mn làm theo cột dọc rồi suy ra nha tks mn

a)

Ta có:

$\left.\begin{matrix}60\\30\\15\\5\\1\\\\ \end{matrix}\right|\begin{array}{l}2\\2\\3\\5\\\\\\\end{array}\qquad\!\!\left.\begin{matrix}280\\140\\70\\35\\7\\1\end{matrix}\right|\begin{array}{l}2\\2\\2\\5\\7\\\\\end{array}\\\Rightarrow \left\{\begin{array}{l}60=2^2.3.5\\ 280=2^3.5.7\end{array}\right.\\\Rightarrow BCNN(60,280)=2^3.3.5.7=840\\\Rightarrow BC(60,280)=B(840)=\left\{0;840;1680;2520;3360;.\!.\!.\!\right\}$

Vậy bội chung của `60` và `280` là $\left\{0;840;1680;2520;3360;.\!.\!.\!\right\}$.

b) Ta có:

$\left.\begin{matrix}84\\42\\21\\7\\1\\\\ \end{matrix}\right|\begin{array}{l}2\\2\\3\\7\\\\\\\end{array}\qquad\!\!\left.\begin{matrix}108\\54\\27\\9\\3\\1\end{matrix}\right|\begin{array}{l}2\\2\\3\\3\\3\\\\\end{array}\\\Rightarrow \left\{\begin{array}{l}84=2^2.3.7\\ 108=2^2.3^3\end{array}\right.\\\Rightarrow BCNN(84,108)=2^2.3^3.7=756\\\Rightarrow BC(84,108)=B(756)=\left\{0;756;1512;2268;3024;.\!.\!.\!\right\}$

Vậy bội chung của `84` và `108` là $\left\{0;756;1512;2268;3024;.\!.\!.\!\right\}$.

c) 

`13` là số nguyên số.

$\left.\begin{matrix}15\\5\\1\end{matrix}\right|\begin{matrix}3\\5\\\\\end{matrix}\\\Rightarrow15=3.5\\\Rightarrow BCNN(13,15)=3.5.13=195\\\Rightarrow BC(13,15)=B(195)=\left\{0;195;390;585;780;.\!.\!.\!\right\}$

Vậy bội chung của `13` và `15` là $\left\{0;195;390;585;780;.\!.\!.\!\right\}$.