September 6th, 2018, 07:20 AM  #11 
Senior Member Joined: Mar 2015 From: New Jersey Posts: 1,462 Thanks: 106  whoops, sorry about the typo. thanks. 15bx20>15 y=15bx35>0 above the line y=15bx35. all points on the line y=43bx which are above the line y=15bx35 also satisfy 15bx35>0. To visualize it, sketch both lines. Point of intersection is at 43bx=15bx35. So both conditions satisfied for x<(39/12b). Last edited by zylo; September 6th, 2018 at 07:57 AM. 
September 6th, 2018, 08:42 AM  #12 
Global Moderator Joined: Dec 2006 Posts: 19,542 Thanks: 1751 
I think 15bx20>15 should have been 15bx20>35 in your initial line. The rest would then need changing as a consequence.
