maths questions....pls help!

2007-12-31 9:46 am
a)平面上有甲組7條平行線與乙組11條平行線,而兩組直線互不平行,這兩組直線可作成多少個平行四邊形?

b)在一個數學測驗中,共有五題選擇題,每題可供選擇的答案有四個。陳同學因為沒有溫習,只好全部五題亂碰答案,求以下概率:

(i) 陳同學全部五題答對;

(ii) 陳同學答對三題或以上。

回答 (3)

2007-12-31 8:11 pm
✔ 最佳答案
(a) any two parallel lines of 甲 and any two parallel lines of 乙 make a parallelogram
C(7,2) × C(11,2)
= 21 × 55
= 1155
so, there are 1155 parallelogram can be formed.

(b) the probability of right answer of every question is 1 / 4
(i) P(x=5) = 1 / 4 × 1 / 4 × 1 / 4 × 1 / 4 × 1 / 4 = 1 / 1024
(ii) P(x≥3) = P(x=3) + P(x=4) + P(x=5)
= C(5,3) × ( 1/4 )^3 × ( 3/4 )^2 + C(5,4) × ( 1/4 )^4 × 3/4 + 1 / 1024
= 10 × 1/64 × 9/16 + 5 × 1/256 × 3 / 4 + 1 / 1024
= 90/1024 + 15/1024 + 1/1024
= 106/1024
= 53/512
參考: 自己
2008-01-02 8:23 am
piika...........
2007-12-31 7:52 pm
a) 7條平行線之間有6個空間, 11條平行線之間有10個空間,所以共有 6 * 10 = 60個平行四邊形

b) i) P(陳同學全部五題答對)
= 1/4 * 1/4 *1/4 * 1/4 * 1/4
= 1/1024

ii) P(陳同學答對三題或以上)
= P(陳同學答對3題) + P(陳同學答對4題) + P(陳同學答對5題)
= 1/4 * 1/4 * 1/4 * 3/4* 3/4 + 1/4 * 1/4 *1/4* 1/4* 3/4 + 1/4 * 1/4 *1/4 * 1/4 * 1/4
= 9/1024 + 3/1024+ 1/1024
= 13/1024


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