21.已知A(0,2)A(0,2)A(0,2),B(1,0)B(1,0)B(1,0),C(3,0)C(3,0)C(3,0),则过AAA,BBB,CCC三点圆的一般方程为 ___x2+y2−4x−72y+3=0{x}^{2}+{y}^{2}-4x-\frac{7}{2}y+3=0x2+y2−4x−27y+3=0___.