1, junior high school mathematics: Is AC the tangent of ⊙D? Guess the relationship between the sum of AB+EB and the size of AC.

Solution: (1)AC is the tangent of ⊙ d.

If DF⊥AC is in F, then in △ABD and △ADF, there are: AD=AD, ∠BAD=∠DAF, ∠DBA=∠DFA, ∴△ABD≌△ADF.

∴ There is BD=DF, and ∵BD is ⊙D radius, so DF is also ⊙D radius, that is, point F is on AC.

∴AC is the tangent of⊙ D.

(2) According to (1), AC is the tangent of ⊙D, with AB=AF.

In △DFC and △EBD, there are ∠ Abd = ∠ DFC = 90, BD=DF, and DE=DC.

∴△DFC≌△EBD has FC=BE.

∫AC = AF+FC

∴AB+EB-AC=AB+EB-AF-FC=0

That is AB+EB=AC.

This question is mainly about congruent triangles's knowledge.