Question

In circle O below, what is the measure of OB?

Answer 1

The circles and chords are just red herrings. All you have here is a right triangle with a hypotenuse of 9 and a side of 4. The length of the other side is:
sqrt(9^2 - 4^2)

Answer 2

use Pythagorean Property..

(hypotenuse)^2 = (base)^2 + (Perpendicular)^2

It's easy..

(9)^2 = (4)^2 + (OB)^2

81 = 16 +( OB) ^2

(OB)^2 = 81 - 16

OB= \[\sqrt{65}\]

Answer 3

APPROX!! 8 should be the answer