Answer:
Finally, substitute AB = 6 and BC = 18 - 2AC in
AB + AC > BC 3rd Triangle Inequality condition
6 + AC > 18 - 2AC
-12 > -3AC
4 < AC
AC > 4
So putting all three results together
AC < 12, AC < 8, and AC > 4
Since AC is less than 8, it's automatically less than 12,
so we can dispense with AC < 12. So we have:
AC < 8 and AC > 4 which can be combined and written as
4 < AC < 8 [that is, AC is between 4 and 8]
