need help for basic trigo question

hi emm i am a begginer new to trigo just learnt it and i need help on this question.

i have included a basic diagram drawn by me.(sry if the diagram is not very nice)

i am stuck with the first part already.2nd part i know how do.

In the diagram,TA and TB are tangents to the circle,centre O at A and B respectively.The radius of the circle is 5cm and AB is 9cm.Find

a)angle AOB

b)the perpendicular distance from O to AB

c)the length of tangent TA

http://i275.photobucket.com/albums/j...g/Untitled.jpg