tolong jawab secepatnya yah..no 16,17 sama caranya

17) x^2 + y^2 + ax - 8y + 4 = 0 => A = a, B = -8, C = 4
Pusat = (a,b) = (A/-2, B/-2) = (a/-2, -8/-2) = (a/-2, 4)
Karena menyinggung sumbu X maka r = b = 4
r = √(a^2 + b^2 - C)
4 = √((a/-2)^2 + (4)^2 - 4)
16 = a^2/4 + 16 - 4
4 = a^2/4
a^2 = 16
a = ± 4 jadi a = -4 atau a = 4