Through the combination of numbers and shapes, the figure is drawn, which shows that the abscissa and ordinate of the center of the circle are equal and both are positive numbers. The radius of a circle is equal to the abscissa and the ordinate.

Therefore, the center of the circle (a, a), a >；; 0, then the radius r=a, so find the distant equation (x-a) 2+(y-a) 2 = a 2, and substitute (2, 1) into the equation (2-a) 2+(1-a) 2 = a.