Assume that formula_10 is not ordinary. Then it goes through at least three points of formula_7. At least two of these are on the same side of formula_14, the perpendicular projection of formula_9 on formula_10. Call them formula_17 and formula_18, with formula_17 being closest to formula_14 (and possibly coinciding with it). Draw the connecting line formula_21 passing through formula_9 and formula_18, and the perpendicular from formula_17 to formula_25 on formula_21 . Then formula_27 is shorter than formula_28. This follows from the fact that formula_29 and formula_30 are similar triangles, one contained inside the another.