March 29, 2017

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Find the point on the line 7x+5y−2=0 which is closest to the point (−6,−2).

  • Calculus - ,

    The idea is to find the family of lines perpendicular to L:7x+5y-2=0.
    Out of that, we find the particular line L1 passing through P(-6,-2).
    The intersection of L and L1 gives the required point (closest to P).

    Family of lines perpendicular to L passing through P0(x0,y0):
    L : 7x+5y-2=0
    L1 : 5(x-x0)-7(y-y0)=0
    in the above, we switch the coefficients of x and y, and change the sign of one of them. To make the line pass through P0, we subtract the coordinates of P0 from x and y respectively.
    For P0(-6,-2),
    L1 : 5(x-(-6))-7(y-(-2))=0
    => 5x-7y+16=0

    Finally, the required point is the intersection point of L and L1, namely, the solution of the system of equations
    7x+5y-2=0 and

    Solving by elimination or by Cramer's rule, we obtain: (-33/37, 61/37)
    Substitute values to verify that the solution is correct:

    L1 :

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