a and b are relatively prime
∴∃ integers x and y such that
ax+by=1 ..........(1)
Let d be any common divisor of ac and b.
since, d|ac,∃ an integer m such that
ac=dm ....(2)
since, d|b,∃ an integer n such that
b=dn ....(3)
Multiplying both sides of (1) by c
acx+bcy=c ............(4)
Putting the values of ac and b from (2) and (3) in (4).
dmx+dncy=c
or d(mx+ncy)=c
∴d|c