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2015-03-09T13:10:01+05:30

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L1:  3 x + y = 4
L2:  x - a y + 7 = 0 
L3:  b x + 2 y + 5 = 0

   L1  ||  L3    =>    the coefficients of x and y in L1 and L2 are proportional or, the slopes of the two lines are equal.
         3 : b  = 1 : 2    =>  b = 6
  L1  Perpendicular to  L2.  The product of the slopes is -1.
           -3 * 1/a = -1    => a = 3

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