Level 1: 2 secant lines (intersecting at an internal point) ad = bc[br]Level 2: 2 secant lines (intersecting at an external point) a(a+b) = c(c+d)[br]Level 3: 1 tangent & 1 secant (intersecting at an external point) [math]a^2[/math] = c(c+d)[br]Level 4: 2 tangents (intersecting at a single external point) a = c

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