Answer:
[tex]m\angle ABO=18^o[/tex]
Step-by-step explanation:
see the attached figure to better understand the problem
we know that
If AB is tangent to circle O at point A
then
AB is perpendicular to AO (radius) and Triangle ABO is a right triangle
so
[tex]m\angle ABO+m\angle AOB=90^o[/tex] ----> by complementary angles in a right triangle
substitute the given value
[tex]m\angle ABO+72^o=90^o[/tex]
[tex]m\angle ABO=90^o-72^o=18^o[/tex]
