GWD1-Q7,Q10,Q17

[myron]

Q7:
A certain university will select 1 of 7 candidates eligible to fill a position in the mathematics department and 2 of 10 candidates eligible to fill 2 identical positions in the computer science department. If none of the candidates is eligible for a position in both departments, how many different sets of 3 candidates are there to fill the 3 positions?
A.42B.70C.140D.165E.315
答案為E

Q10:
What is the value of (x - y)4?
(1)The product of x and y is 7.
(2)x and y are integers.
答案為C

Q17:
If x, y, and k are positive numbers such that (X/(X+Y))(10) + (Y/(X+Y))(20) = k and if x < y, which of the following could be the value of k?
A.10B.12C.15D.18E.30
答案為D

頭快爆炸了....請教以上問題.

[micht]

#7
C(7,1)*C(10,2) =315

#10
(C) 因為 (1/7)(49)=7

XY=7
可為(1, 7) or (7,1)
(-1,-7) or (-7, -1)


怪怪 代回去 (x - y)4=-24 or 24 那不是仍無法得知是哪一個

[吳志道]

Q7 C(7,1)(10,2) =315
Q10 (1) xy 相乘 7 (迷二 可能有分數)
(2) 由(1)知 (x-y)^ =6^4 因為7 =(-1,-7)(7,1) =(x,y) 也可以互為
==> C 充分
Q17 (X/(X+Y))(10) + (Y/(X+Y))(20) = k
==>(1)10 +10Y/X+Y
(2)當x+y近似Y (因為X粉小) 10+10y/y ==>10+10=20
(3)當x+y->2y(因為x趨近y) 10+10y/2y 10+5=15
(4)15<10 +10/X+Y <20

[fpeipei]

#7.
從7人中選1人，另從10人中選2人，且不可重複
所以C(7,1)*C(10,1)=315

#10.
(1)X*Y=7 且 (2)X,Y為整數
所以X,Y=(1,7)(7,1)(-1,-7)(-7,-1)
得到(X-Y)^4=1296

[fpeipei]

呵呵....好像搶答喔... :D

[micht]

原來題目是 ( )^4 害我以為是( )x4 =_=!! 大家真熱心

[myron]

sorry!偶還是不了Q17.
可否再幫幫忙~~

[micht]

(X/(X+Y))(10) + (Y/(X+Y))(20) = k and if x < y

10X+20Y=K(X+Y)

X<Y
X=2, Y=3 (20+60)/5=18

偶的解法比較笨...
晚點再來研究 吳大哥的解法

誰幫我解一下那題parking lot的題目阿
http://www.formosamba.com/phpBB3/viewtopic.php?t=923

[吳志道]

[吳志道]

(X/(X+Y))(10) + (Y/(X+Y))(20) = k
==>(1)10 +10Y/X+Y
(2)當x+y近似Y (因為X粉小) 10+10y/y ==>10+10=20
(3)當x+y->2y(因為x趨近y) 10+10y/2y 10+5=15
(4)15<10 +10/X+Y <20

以上哪個步驟不懂 再提喔 ^^ 拍謝 之前打錯 QQ

[homaru]

(X/(X+Y))(10) + (Y/(X+Y))(20) = k
約分之後-->10+(10Y/(X+Y))=k
10Y/(X+Y)=k-10
代入k
A,k=10,so 10Y/(X+Y)=0....不合
B,k=12,so X=(1/4)X....不合
C,k=15,so Y=X....不合
D,k=18,so Y=(9/4)X
E,k=30,so Y=-2X....不合

[myron]

不是很了....WHY約分之後會=10+(10Y/(X+Y))=K
SORRY~我腦筋還轉不過來.

[homaru]

(X/(X+Y))(10) + (Y/(X+Y))(20) = k
10X/(X+Y)+20Y/(X+Y)=k
(10X+10Y+10Y)/(X+Y)=k
[10(X+Y)+10Y]/(X+Y)=k
10 + 10Y/(X+Y)=k

[魚]

呵呵....好像搶答喔... :D

這個小太陽好口愛 (*)

[myron]

我真是太...呆了!
多謝了,各位.