dse數學問題

以下問題都已知答案,但我不明白為何是這些答案,

希望數學勁人幫幫忙,詳細解說一下,謝謝!>,<"

Q1)

http://upload.lsforum.net/users/public/w2876P290(Q...

ans. is C, but why B is not??

Q2)

http://upload.lsforum.net/users/public/z16858P302(...

ans. is A, but why B is not??

Q3)

Emily puts 6 pens into 3 different pencil cases.

In how many ways can she put the pens?

ans. is 729.

Q4)

8 boys and 5 girls are nominated for 4 different posts in a student association.

How many ways are there to select the students?

ans. is 17160.

Q5)

There are 8 bags on a table. 5 of them are empty and each of the other bags contains one ball in it. Andy puts a ball into a bag at random. If a bag is chosen afterwards, the probability of getting an empty bag is

ans. is 35/64.

Q6)

http://upload.lsforum.net/users/public/l20875P70(Q...

ans. is A.

Q7)

http://upload.lsforum.net/users/public/c30105P70(Q...

ans. is A.

Q8)

Which of the following figures cannot be used to tessellate a plane completely?

A. Parallelogram

B. Isosceles triangle

C. Regular hexagon

D. Regular pentagon

ans. is D.

Q9)

http://upload.lsforum.net/users/public/e9885P73(Q2...

ans. is D.

Q10)

http://upload.lsforum.net/users/public/w15768P128(...

ans. is C.

Q11)

In triangle ABC, the coordinates of the mid-points of the three sides are (1,5),(2,4) and (4,6) respectively. Which of the following points is a vertex of the triangle?

A. (-1,4)

B. (0,3)

C. (3,7)

D. (5,6)

ans. is C.

Q12)

http://upload.lsforum.net/users/public/i53040P189(...

ans. is D.