As you can see from the derivation above, the centripetal force is derive from two equation. F = ma and a =v^2/r. The confusing part for most of the student would be the acceleration and how it came to be v^2 /r. We can further understand that Va and Vb are close to each other thus we are able to assume that, the resultant velocity linear. Look at the explanation below
After we have discovered the linear velocity, the derivation of centripetal force is SUPER easy