It is sometimes convenient and intuitive to solve the problem of circle by geometric method. And the calculation amount is small.

Set point P(2, 1), and the radius of the circle is1; The intersection of the circle and the positive semi-axis of the Y-axis is (0, 1), which is a tangent point in the tangent line passing through point P, and is set as A(0, 1), and the other tangent point B is in the fourth quadrant.

The slope of op is kop =1/2;

OP vertically divides AB,

So the slope of AB KAB =-2; (Kop *Kab=- 1)

So the equation of AB is: y- 1=Kab(x-0)=-2x point oblique type.

Namely: y=-2X+ 1.