GWD Math-9 Q36

fudanwyh · 发表于 2010-11-15 06:44:21

Q36:

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.10

B.12

C.15

D.18

E.30

lpilikeit · 发表于 2010-11-15 08:57:13

那个10 和 20是一个和其他项相乘的乘数还是指数？

sdcar2010 · 发表于 2010-11-15 10:18:18

(x/(x+y))(10) + (y/(x+y))(20)
= (x/(x+y))(10) + (y/(x+y))(10 + 10)
= ((x+y)/(x+y))(10) + (y/(x+y))(10)
= 10 + (y/(x+y))(10)
= k

if x < y, then 1>y/(x+y) > 0.5, then 20 >k > 15

fudanwyh · 发表于 2010-11-15 14:23:08

谢NN！
果然是个温暖且强大的网站阿！

terryhan · 发表于 2010-11-20 20:57:28

因为：x<y, 所以limit [y/(x+y)]趋向于1。

GWD Math-9 Q36

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