难的初二数学

口袋中有20个球，其中白球9个，红球5个，黑球6个．现从中任取10个球，使得白球不少于2个但不多于8个，红球不少于2个，黑球不多于3个，那么上述取法的种数是（ ）．
A. 14 B.16 C.18 D.20

World's Hardest Easy Geometry Problem
Using only elementary geometry, determine angle x. Provide a step-by-step proof.

You may only use elementary geometry, such as the fact that the angles of a triangle add up to 180 degrees and the basic congruent triangle rules (side-angle-side, etc.). You may not use more advanced trigonomery, such as the law of sines, the law of cosines, etc. There is a review of elementary geometry below.

This is the hardest problem I have ever seen that is, in a sense, easy. It really can be done using only elementary geometry. This is not a trick question.

Here is a very small hint. Here is a small hint.

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World's Second-Hardest Easy Geometry Problem
Using only elementary geometry, determine angle x. Provide a step-by-step proof.

This is a variation of the problem above. This is also a very hard problem that is, in a sense, easy.

Here is a very

1.已知平行四边形ABCD的周长为52，自顶点D作DE垂直AB于E，DF垂直BC于F，若DE=5，DF=8,求BE+BF的长？
2.已知a,b均为正整数，且a+b=2,求p等于a的平方加4的和的算术平方根与b的平方加1的和的算术平方根的最小值

如图,以知E为平行四边形ABCD中DC边的延长线上一点,且CE=DC,连接AE分别交BC,BD于点F、G.
(1)求证:△AFB≌△EFC；
(2)若BD=12cm，求DC的长.

图：有一个平行四边形，向左边倾斜，从右上角顺时针为A B C D,连接BD，并延长DC,使CE=CD，连接AE

注意哦，是延长DC，不是延长CD,

第（1）超简单，第（2）有点难

平行四边形ABCD中 点E在BC中点 连接AE DE 以知AE=BE=CE 求角AED的度数

一张正方形纸ABCD边长为4 将CD沿EF对折 点C落到AB中点。求折痕EF的长
问题补充：点E在AD上点F在BC上