Data Sufficiency

How do you relate the product of the x's and y's to the difference in the y's/ the difference in the x's?

tlt2372 wrote:How do you relate the product of the x's and y's to the difference in the y's/ the difference in the x's?

when solve this problem, i dont use the point (4,3)... dunno why they have it here but here is the way i solve this
L: y=ax+b, y-intercept=b x_intercept=-b/a ( when y=0 x=-b/a)
K: y=cx+d y-intercept=d and x-intercept=-d/c
1. the product of x-intercepts of L and K >0 or bd/ac>0
this can happen when bd>0 and ac> 0 or opposite
thus insufficient
2. bd>0 but we know nothing about ac while the final conclusion is about ac
so insufficient
(1+2) bd>0=> ac>0 because bd/ac>0
C

This thread had been discussed before. If you need more explanation, check this link :

https://www.beatthegmat.com/gmat-prep-ds ... tml#305514