Say the two lines are
L: x/a + y/b = 1 and K: x/c + x/d = 1
Slope of L = -b/a
Slope of K = -d/c
Product of slopes = bd/ac
Statement 1: ac>0 - Insufficient
Statement 2: bd<0 - Insufficient
Combined - bd/ac is negative.
I reasoned in this way:
we must show whether be/ad<0
1. y=c/a and y1=k/d; ck/ad>0 where c and k are the constants. it doesn't say much
2. ck/be<0 alone insuff
together it's clear that ad and be have different signs so c is suff
anyway, it took me far more than 2 minutes to solve this one and would like to know if there is any shortcut. moreover, why does the argument tell us that the intersection point is 4,3?