Salah satu persamaan garis singgung lingkaran :

1) y = 7 – 2x
=> m = -2
Karena tegak lurus maka
m . (-2) = -1
=> m = 1/2

2) x² + y² + 6x – 4y – 7 = 0
a = 6 , b = 4
Pusat : (a, b) = (6/-2, -4/-2) = (-3, 2)
Jari-jari : r = √((-3)² + 2² – (-7))
=> r = √(9 + 4 + 7)
=> r = √20
=> r = 2 √5

3) Persamaan garis singgung lingkaran bergradien m = 1/2
(y – b) = m(x – a) ± r √(m² + 1)
(y – 2) = (1/2) (x – (-3)) ± 2√5 √((1/2)² + 1)
(y – 2) = (1/2)(x + 3) ± 2√5 . √((1/4) + 1)
(y – 2) = (1/2)x + (3/2) ± 2 √5 . √(5/4)
(y – 2) = (1/2)x + (3/2) ± 5
(Kedua ruas kali 2)
2y – 4 = x + 3 ± 10
0 = x – 2y + 7 ± 10
Persamaan garis singgung lingkaran 1
x – 2y + 7 + 10 = 0
x – 2y + 17 = 0
x – 2y = -17
Persamaan garis singgung lingkaran 2
x – 2y + 7 – 10 = 0
x – 2y – 3 = 0
x – 2y = 3