Inequalities gives me hard time !!

[aru4912]

Folks,

I am very sure that I am missing some very basic concepts while solving inequalities questions such as the below one:

 

If M>0 and N>0, is (M+X)/(N+X) > (M/N) ?

 

A. (M

B. (X>0)

 

I solved as below:

= >Is (M+X)/(N+X) > (M/N)

Cross multiplying => NM+NX > MN+MX

=> NX > MX

Rephrasing the qn. as => is N > M ?

 

From choice A, it is true that N > M. So I chose A which was not the

answer.

 

Can someone please help me where I went wrong ?

 

 

Thanks,

A B

[J2010]

If M>0 and N>0, is (M+X)/(N+X) > (M/N) ?

 

A. (M

B. (X>0)

stmt 1: even though m

stmt 2: even though x>0, the inequality depends on whether m

 

together, the inequality holds true and hence answer is C.

 

Let me know if my approach is incorrect.

[venkat007]

From the condition we know both M and N are +ve. what we have to find is

X(N-M)/N(N+X) >0 ?

From 1) we know that N-M>0

but still the value of the inequation depends on the value of X.

a) If X is negative and whose absolute value is less than N. Then the result will be less than 0.

b)If X is positive and its absolute value is greater than N . then the result will be greater than

so Insufficient

 

From 2)X>0 . but here the result is dependent on the value of N-M .

Insufficient.

 

Combining the above .

we can say that the result is positive.

Ans ) C

[12rk34]

Dear aru4912,

The problem with DS questions is that you cannot solve them like PS questions. So intead of using the statements to prove or contradict what has been asked for, if you use the question to prove or contradict the statement(s), you will get wrong answers.

In problems like this one, the better approach is to put possible values and see whther it proves or contradicts the expression.

(1) let M=1, N=2, X=3

Then M+X/N+X=4/5>M/N(=1/2)

But if M=1, N=2, X=-1

M+N/M-N=0

(2) X>0 If M=2, N=1, X=1

Then M+X/N+X = 3/2

Combining these two, the expression is correct. Hence C.

NOTE: So please don't use the expression to prove or contradict the statements for DS problems.:)

[aru4912]

Friends- you are all right. OA is C. Lesson learned : please don't use the expression to prove or contradict the statements for DS problems (thanks 12rk34)