Ф(x) = kQ/x

Ф1 = Ф(x1) = kQ/x1

Ф2 = Ф(x2) = kQ/x2

Ф3 = Ф(x3) = kQ/x3

Ф2 - Ф3 = Ф1 - Ф2

kQ/x2 - kQ/x3 = kQ/x1 - kQ/x2

1/x2 - 1/x3 = 1/x1 - 1/x2

1/x3 = 2/x2 - 1/x1

1/x3 = 2/2 - 1/1 = 1-1 = 0

1/x3 = 0

x3 = беск.