x²+y²+Dx+Ey-4=0
=>(x+D/2)²+(y+E/2)²=4+D²/4+E²/4
∴圆心为(-D/2,-E/2),即1=-D/2,-2=-E/2
=>D=-2,E=4
∴半径r²=4+D²/4+E²/4=4+1+4=9
=>r=3