如图,四边形ABCD内接于⊙O,AB=AD,过A点的切线交CB的延长线于E点.求证:AB2=BE•CD.

问题描述:

如图,四边形ABCD内接于⊙O,

AB
=
AD
,过A点的切线交CB的延长线于E点.求证:AB2=BE•CD.

证明:连接AC,
∵EA切⊙O于A,
∴∠EAB=∠ACB.

AB
=
AD

∴∠ACD=∠ACB,AB=AD.
于是∠EAB=∠ACD.
又四边形ABCD内接于⊙O,
∴∠ABE=∠D.
∴△ABE∽△CDA.
于是
AB
CD
=
BE
DA
,即AB•DA=BE•CD.
∴AB2=BE•CD.