Question 201803
Very  good  question.   MOST  people  have  a  great  deal  of  difficulty  with vertical  and  horizontal  lines.
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 In  short,  Vertical  lines  have  form,,,x=#
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,,,,,,,,,,,,,Horizontal  lines  have  form,,,,y=#
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Remember  general  linear  equations  have  form,  y = mx +b
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Horizontal  lines,  reason  as  follows: m=0, therefore,  for  any x,,(x*0=0), y=b  
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Vertical  lines,are  not  as  reasonable.  Books  say  to  stay  away  from  y=mx +b  for  vertical  lines.  Lets  do  it  this  way. For  any y, x=#.  This  is  the  only  form  that  allows y  to  vary,  and  x  remain  constant,( like 0*y = x+b,,,,but  mathematically  not)
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If  a  VERTICAL line  is  coordinated, it  is  (y1,x),(y2,x) (y3,x),etc: hence x=#,  for  all y"s
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If  a  HORIZONTAL line  is  coordinated, (y,x1),(y,x2), (y,x3), etc: hence y=#  for  all x"s
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When  it  is  all  done, perhaps  the  best  way  to  remember  is  that either y=# or x=#,  and  pick  the  one  that  works.