preetamsaha
From any two numbers x and y, we define x* y = x + 0.5y - xy. Suppose that both x and y are greater
than 0.5. If x*x<y* y then which of the following must be true :
A. 1 > x > y
B. x > 1 > y
C. 1 > y > x
D. y > 1 > x
E. none of the above
A faulty question...
Let us remember that it is 'MUST be true"question...Now, x#x<y#y ( It is always better to mention that you are defining a function, and take it something different from normal mathematical operations, such as +, -, * etc.)
\(x#y = x + 0.5y - xy\)....
(I)\( x#x = x + 0.5x - x*x=1.5x-x^2\)....
(II) \(y#y = y + 0.5y - y*y=1.5y-y^2\)....
GIVEN that x#x<y#y.............1.5x-x^2<1.5y-y^2..........x^2-y^2+1.5y-1.5x>0.........(x-y)(x+y)+1.5(y-x)>0........(x-y)(x+y-1.5)>0
Two cases, as also shown in posts above
Case I :- x>y, then x+y-1.5>0 or x+y>1.5
>1.5 could mean x+y=1000 too.
So, in this case, MUST be true is \(x>y>0.5\)
Case II :- x<y, then x+y-1.5<0 or x+y<1.5
But the least value of x+y>1, so 1<x+y<1.5
So, in this case, MUST be true is \(0.5<x<y<1\)
The correct answer should encompass any of the two cases, then only must be true will hold.Now let us look at the choices. None is encompassing any of the above two cases, so logically it should be E
But let us check the OA given..
Again remember we are looking at MUST be true..
B. x>1>y
If x=2, y=1.5......x>y and x+y>1.5.
So, x>1>y need not be true.
Now, when could be B the answer
If question was..
Quote:
For any two numbers x and y, we define x#y = x + 0.5y - xy, and both x and y are greater
than 0.5. which of the following options will always give x#x<y#y?
Here any values of x and y in the range x>1>y will always give x#x<y#y.