我將３個坐標轉成３條式...然後就沒辦法計算 1. 9+3E+F=0 2. 26-D+5E+F=0 3. 20+2D+4E+F=0?

(a)
設圓心為 (x,y) , 半徑為 r
r^2 = x^2 + (y-3)^2 = (x+1)^2 + (y-5)^2 = (x-2)^2 + (y-4)^2
每一項皆減 x^2+y^2 , 等式恆成立:
- 6y + 9 = 2x - 10y + 26 = - 4x - 8y + 20

- 6y + 9 = 2x - 10y + 26
4y = 2x + 17

- 6y + 9 = - 4x - 8y + 20
2y = - 4x + 11
4y = - 8x + 22

4y = 2x + 17 = - 8x + 22
x = 1/2 = 0.5
y = (2x+17)/4 = (1+17)/4 = 9/2 = 4.5
r^2 = x^2 + (y-3)^2 = 0.5^2 + (4.5-3)^2 = 1/4 + 9/4 = 10/4 = ( √10 / 2 )^2
故圓方程式為 ( x - 1/2 )^2 + ( y - 9/2 )^2 = ( √10 / 2 )^2 ..... Ans

(b)
d( (0.5,4.5) , (1,2) )
= √ ( 0.5^2 + 2.5^2 )
= √ 6.5
= √26 / 2
≠ √10 / 2
故 (1,2) 不在此圓上 ..... Ans