## division

I have the following :

1. $a_{1} | a$
2. $b_{1} | b$
3. $a_{1}b_{1} | ab$
4. $a_{1}b_{1} | k$
5. $a | k$
6. $k | ab$
7. $k | aa_{1}b_{1}$

I need to show that $k=ab_{1}$.
I try to show that $k | ab_{1}$ and $ab_{1} | k$, but I dont know how to show this.