Помогите по геометрии 9 класс

1)
MB * MA = MC * MD
13 * 5 = (8 + MD) * MD
MD^2 + 8 MD - 65 = 0
MD = 5
MC = 8 + 5 = 13

2)
6 * 2 = (x+1) * x
x^2 + x - 12 = 0
x = 4

3)
AM * CM = MB * MD
24 * 8 = 4a * a
a = v48 = 4v3
4a = 16v3