[DSE數學2018] Maths 討論區 (2)

咁又太多啦

其實你㩒下機已經可以得知
正確答案係你個答案嘅 5/2 倍

當 ABCDA (先唔理 suit) 係你分析入面嘅其中一個 Case
5!/2! 係 AABCD 所有排列嘅數目
如果你將你個原先嘅答案乘以 5!/2! 嘅話
就會由呢個 Case 衍生埋類似 BCDAA CDABA 呢類排列
但係呢類排列用你個方法其實已經數咗
所以就咁乘以 5!/2! 嘅話就會重復咗

In your previous computation,
you only count the pattern likes

ABCDA
BACDA
BCADA
BCDAA

In other words, you only count the pattern which the repetition always appears at the end.

Number of such patterns = 4!x4 .......(i)

On the other hand,
Total number of ways to arrange {A,B,C,D} in 5 seats such that exactly one repetition occurs:
= 5P5/2P2 X 4
= 4X5!/2 ...... (ii)

Divide (ii) by (i), you wiill get 5/2

Fix a letter, say A

_ _ _ _ A

Number of ways to put {A, B, C, D} in the above seats = 4P4

But we also have something like

_ _ _ B

_ _ _ C

_ _ _ D


Your calculations only count such patterns.
Total number of them = 4P4 x 4


On the other hands, we want to know how many ways to permute the following 4 different types of patterns:

(p) AABCD
(q) ABBCD
(r) ABCCD
(s) ABCDD

Each type has 5P5/2P2 ways to permute

So, total number of them = 5P5/2P2 x 4

只係用到排列同組合
無話唔可以出
但當然算係超難啦

個人強烈建議用巴打呢個方法做，
同我上面講2014 dse果題差唔多，
易明又少機會錯

最煩係6-8月應該會好少生意。
同埋而家學生鍾意補習老師後生，
好似好難長做。

機械鍵盤原來人手砌，仲可以成為一門生意，真係長知識了

2016 5**留名
樂於解答

想請問下，如果用希羅公式，用不用寫 s = (AB+AC+BC)/3 個d 算式出來？ 我可不可以直接寫透過希羅公式，ABC的Area 是 xx cm^2 ? 會不會扣step分?

打錯 即是不可以直接寫透過希羅公式，ABC的Area 是 xx cm^2 ?？

唔得。
以往ce考過,
佢要你寫條formula出黎，同埋要定義清楚s先會比分。

因為你第一個係52/52，姐係乜都得
而排次序既時候，乜都得係唔洗排
所以唔係5！/2！而係4！

仲想寫少d字，多d 時間可以check paper

mc dse past paper難度點排

我想問下應該點樣計 要用乜fomula?