AB is the diameter andTB is a tangent to a circle. if Q is a point on TB then find QA

 Your question is not clear and appears to be incomplete.

From the given data , you can find the QA by applying Pythagoras theorem in it as ∠ABQ = 90 °.(The tangent is always perpendicular to the radius at the point of contact .)

QA  = √(AB²+QB²)

Hope this would have cleared your doubt .