1. condition 1: 在X轴上的截距之乘积为正 => either both lines intersects the x-axis to the right of the origin or both to the left of the origin. condition 2: 在Y轴上的截距之乘积为负 => one of the line intersects the y-axis above the origin and the other below the origin try drawing a graph?
2. A/B > C/D => (A/B) *B*D > (C/D) *B*D (as long as B*D are both greater than 0, otherwise you have to reverse the >)