PP-T2-DS152

[Huang Hsin-Yi]

152. 15540-!-item-!-187;#058&010751
A certain jar contains only b black marbles, w white marbles, and r red marbles. If one marble is to be chosen at random from the jar, is the probability that the marble chosen will be red greater than the probability that the marble chosen will be white?

(1)r/b+w>w/b+r

(2)b-w>r

Ans: A

我想它應該是要問 r-w>0,但想到這就堵住了

[小花]

(1)
想辦法把分母消掉 同乘(b+w)(b+r)
答案就很明顯了

(2)b>r+w
還是不知 r 和w 的關係

[Huang Hsin-Yi]

小花大大,拍細
我把(1)選項通分後 分子變成

r(b+r)>w(b+w)這樣我還是看不出來

能不能再請你指點我一下

[FIG]

移過來
(r2-w2)-b(r-w)>0
因為r,w,b都是正數
所以該試成立，基本上就是 r>w

[Huang Hsin-Yi]

原來要移過來呀! 謝謝囉