ax + b = 0

relaxin99 wrote:ax + b = 0, is x>0?

(1) a + b > 0
(2) a - b > 0

The anwer is E, i thought it is C. can someone please explain

I fell in to that trap as well on first glance. But here's the best I can do:

ax + b = 0
ax = -b
a = -b/x

1) Since a + b > 0, and ax + b = 0, then (x-1)*a must be negative. That doesn't help us determine the sign of x: all we know is that either (x-1) < 0 or a < 0.

2) a > b. I tried to get something useful out of it but I couldn't; this may be a failure on my part.

1) & 2) a > 0. This still only implies that bx < 0, which isn't sufficient to show that x > 0.


I'm not convinced by my answer, but it's the best I've got. Hopefully someone will elaborate.

X= -b/a

Is -b/a > 0

St1 ) a>-b INSUF ( 3>-2 or 3>2)

St2) a > b INSUF ( 1 > -2 or 2 > 1 )

St1+2) 2a>0 and a>0

we still don't know whether b is negative or positive

E

logitech wrote:X= -b/a

Is -b/a > 0

St1 ) a>-b INSUF ( 3>-2 or 3>2)

St2) a > b INSUF ( 1 > -2 or 2 > 1 )

St1+2) 2a>0 and a>0

we still don't know whether b is negative or positive

E

agreed same approach..