Maths problem:為何我計多次（a)也是（-1,-1）?

Let centre=(h,k)
As centre lies on x-axis, k=0
∴ centre=(h,0)So, eqt. of circle: (x-h)²+y²=r² - - - - - (#)

From A(-1,1)&B(0,-2) :
{ (-1-h)² + 1² = r²
{ (0-h)² + (-2)²= r²
{ (1+h)² + 1 = r² . . . . . . .. . . ①
{ h² + 4 = r². . . . . . . . . . . . . ②
①-②: (1+2h)-3 = 0
　　　2h-2=0
　　　∴ h=1

So, centre=(1,0)

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(#): Initially, eqt. of circle should be: 
(x-h)²+(y-k)²=r²
Now, centre lies on x-axis, k=0
∴ eqt. of circle: (x-h)²+y²=r²