DS Exponents

rainbownlife · Posted: Tue Mar 25, 2008 3:22 pm Post subject: DS Exponents

Is 9^x + 9^(-x) = b ?

(1)x > 0 = b

Is is a sufficient condition?

Here is my working:
putting value of b we get
Is 9^x + 9^(-x) = 0 ?
=> 9^x + 1/9^x = 0
=>( 9^2x + 1 ) /9^x = 0
This will be true only if 9^x (is not equal to) 0
and 9^2x = -1 (this is not possible as even power cannot be -ve)
and 9^x cannot be 0.

Hence this provides answer NO to the question and should be sufficeint.
(But the answer is NOT Sufficient)

Can someone please explain where i am going wrong ?

thanks.

Stuart Kovinsky · Posted: Tue Mar 25, 2008 4:22 pm Post subject:

Just to clarify, is statement (1) actually:

x > 0 = b

That's a really weird statement, don't think I've ever seen anything like it before on the GMAT.

If the statement is correct, then I agree with your conclusion that there's no way that

9^x + 9^(-x) = 0

since that would mean that

9^x = -(1/9^x)

and 9 raised to any power is going to be positive.

rainbownlife · Posted: Wed Mar 26, 2008 8:42 am Post subject:

Hi Stuart
This is question 142 from official guid 11 (DS section).
Yes it is printed as x>0=b
Dont have the book at this moment with me but as far as i remember, the explanation uses picking values to solve and I wanted to understand mathematically.

can x>0=b be a printing mistake? (i am using edition 11)
OG answer cannot be wrong? can it?