Sorry, I have to use English as typing.

Classification of quadrilaterals.(We say yes for Isomorphism)

Convex quadrilateral ABCD:

<A + <B +<C +<D = 360, 0< <A,<B,<C,<D < 180

1)<A=<B=<C=<D=90 2)<A=<C=90,<B>90,<D<90(or<D>90,<B<90) 3)<A=<B=90,<C>90,<D<90 (or<C<90,<D>90) 4)<A=90,<B>90,<C>90,<D<90 5)<A=90,<B>90,<C<90,<D>90(or omit) 6)<A,B,C>90,<D<90,7)<A>90,<B,C,D<90.

Concave quadrilateral ABCD:

<A+<B+<C+<D=360, 0< <A,<B,<C,<D <360

1) <A>180,<B>90,<C,<D <90 2)<A>180,<C>90,<B,<D<90 3)<A>180,<B=90,<C,<D,<90 4)<A>180,<C=90,<B,<D <90 5)<A>180,<B,C,D <90.

I wrote this is the same as last one, but a little clearly.I wish it is right.

It is not good that I can't copy any words as usual.I can't save time.

所有跟帖： 

• 谢 津 的进一步探索。和许多问题一样，四边形分类可能有许多方法来解决，您的方法和我的不一样。 -皆兄弟也- ♂ (782 bytes) () 10/17/2010 postreply 12:11:36