If x, y, and k are positive numbers such that (
A. 10
B. 12
C. 15
D. 18
E. 30
我的算法是(
you can get y/x=(k-10)/(20-k); since x<y, y/x>1
so, (k-10)/(20-k)>1, (k>0)
let k<20, you will get k>15
otherwise, k>20 contradicts to the result of k<15;
18 is the right choice here
还有什么疑惑的,你不是明白了吗?
k=10+(10Y/X+Y),因为x<y,
所以Y/(X+Y)的值域为(1/2,1),所以10+10Y/(X+Y)值域为(15,20),因为值域是开区间而不是
闭区间的,所以取不了15跟20两个值,所以符合条件的就只有18了
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