JEE Main & Advanced Mathematics Straight Line Question Bank Slope of line, Equation of line in different forms

  • question_answer
    The point \[P\,(a,\ b)\]lies on the straight line \[3x+2y=13\] and the point \[Q\ (b,\ a)\] lies on the straight line \[4x-y=5,\]then the equation of line PQ is                                                     [MP PET 1999]

    A)            \[x-y=5\]                                  

    B)            \[x+y=5\]

    C)            \[x+y=-\ 5\]                             

    D)            \[x-y=-\ 5\]

    Correct Answer: B

    Solution :

               Point \[P(a,b)\]is on \[3x+2y=13\]                    So, \[3a+2b=13\]                                             .....(i)                    Point \[Q(b,a)\]is on \[4x-y=5\]                    So, \[4b-a=5\]                                                    .....(ii)                    By solving (i) and (ii), \[a=3,b=2\]                    \[P(a,b)\to (3,\,2)\]and \[Q(b,a)\to (2,\,3)\]                    Now, equation of PQ \[y-{{y}_{1}}=\frac{{{y}_{2}}-{{y}_{1}}}{{{x}_{2}}-{{x}_{1}}}(x-{{x}_{1}})\Rightarrow y-2=\frac{3-2}{2-3}(x-3)\]                    Þ \[y-2=-(x-3)\Rightarrow x+y=5\].


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