Locus of Mid-point

Geometry Level 3

Consider two points P ( a , b ) P(a, b) and Q ( a b , 4 a + b ) . Q(a-b, 4a+b). If point P P moves along the line y = x , y=-x, what is the equation of the locus of the mid-point of P Q ? \overline{PQ}?

2 x y = 0 2x-y=0 2 x + 3 y = 0 2x+3y=0 x + 2 y = 0 x+2y=0 2 x 3 y = 0 2x-3y=0

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1 solution

Tom Engelsman
Mar 24, 2016

Let the midpoint of PQ be expressed as:

((a + a-b)/2, (4a+b + b)/2) = ((2a-b)/2, (4a+2b)/2) (i)

If the point P(a,b) moves according to the line y = -x, then b = -a. Substituting this value into (i) yields:

(3a/2, a)

and we obtain the set of parametric equations:

x = 3a/2, y = a

which results in y = 2x/3 => 2x - 3y = 0.

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