Without needing the distance formula
from 4,1 to 0,1, you move the "x" from 4 to 0, over the same "y" value of 1, so is just 4 units long, from the center to an endpoint, and the chord that stems from the center to an endpoint is the radius, so r = 4
from 4,1 to 0,1, you move the "x" from 4 to 0, over the same "y" value of 1, so is just 4 units long, from the center to an endpoint, and the chord that stems from the center to an endpoint is the radius, so r = 4