Question 1400
 Given a system of two linear equations:

 a1 x + b1 y = c1...(1),

 a2 x + b2 y = c2...(2)

 slope of (1): m1 = -a1/b1 (if b1 <> 0)

 slope of (1): m2 = -a2/b2 (if b2 <> 0)


 if a1/b1 = a2/b2 (same solpe) but is not equal to c1/c2 then the two lines
 are parallel

 if a1/b1 * a2/b2 = -1 (ie a1a2 = -b1b2 ) then the two lines
 are perpendicular.

 Try to think when b1 or b2 = 0