题库搜索>问题求解PS-12786
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?
分析A选项
分析B选项
分析C选项
分析D选项
分析E选项
所以选D
256657dkeo
看到which….could be the value of x,其实x肯定是个范围值,如果问的是准确值就会这样问what is the value of x.只要题目意思是在问范围值基本上都有简单方法(看你时间,你时间还很充足,可以一步一步化简,如果很急,就另辟蹊径);看题干K=…..,那么就是这一堆肯定有个范围,也就是在问你(x/(x+y)(10)+(y/(x+y))(20)的取值范围(求不等式),你列出在白班上就很清晰了,2个部分都有x/(x+y)和y/(x+y),x0
0 回复 2022-05-23 17:48:25
256657dkeo回复256657dkeo
这2个部分都小于1,那么整体肯定小于30,然后假设x=y,那么值为15,这样这个区间就有可能在2个范围内,一个15—30,一个小于15,看一下选项,可以很大概率去赌一把18(因为答案只有1个);如果要稳那就把式子后半裁成2个相等的式子(y/(x+y))(10),一个和前面相加等于10,后面这个肯定大于5,所以得答案
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2022-05-23 17:51:49
Kath1230
先通分 在带入试一下。。。否则考场哪有时间。。。
默默
可以约等于,反正就是求个大概值
默默回复默默
直到化到最简为止
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2018-05-08 04:49:09
默默回复默默
首先, y=x 的话,是15,他肯定大于15
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2018-05-08 04:55:10
默默回复默默
找到另外一个区间,没有x时,取中间值
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2018-05-08 04:58:08
shinging1031
請問10+10y/2y=15是怎麼算出來的?
默默哒回复shinging1031
先通分,提取公因式,再约去同分子分母 得出: [10(x+y)+ 10y]/ x+y = k 10 + (10y)/(x+y) , 因为 x< y, 设x=4 y=6 , 代入 则 k=16 ; 如果再设 x=0 , y=10 , 代入则k=20 所以 k 的取值范围是在 16- 20 之间, inclusive。
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2018-02-17 17:09:28
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