Jika lingkaran x^2 + y^2 +6x +k=0 dan x^2+y^2+8y -20=0. Saling bersinggungan dalam. Tentukan nilai k

x² + y² + 6x + k = 0
(x + 3)² + (y - 0)² = 9 - k
P1 = (-3,0)
r1 = √(9 - k)

x² + y² + 8y - 20 = 0
(x - 0)² + (y + 4)² = 20 + 16
P2 = (0,-4)
r2 = √36 = 6

P1P2 = √((-3 - 0)² + (0 - (-4))²) = 5

P1P2 = r2 - r1
5 = 6 - √(9 - k)
√(9 - k) = 1
9 - k = 1
k = 8