It’s weird to picture, but every point on the circle has a tangent vector which is, by definition, at a right angle to the radius vector, which is what the sides of that larger pie piece both are.
You have to think of the infinitesimally small point where the two meet.
I’m not buying that a curved lined emerging from a straight line is ever at a 90° angle to the straight. Not at any value greater than zero, anyway.
It’s weird to picture, but every point on the circle has a tangent vector which is, by definition, at a right angle to the radius vector, which is what the sides of that larger pie piece both are.
You have to think of the infinitesimally small point where the two meet.