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Paper 1 · June 2019 · Coordinate geometry

Find, in terms of kk, the equation of the perpendicular bisector of the line joining A(2k,k)A(2k, k) and B(4k,9k)B(4k, 9k).

A4x+y=17k4x + y = 17k
Bx−4y=−17kx - 4y = -17k
Cx+4y=23kx + 4y = 23k
D4x−y=7k4x - y = 7k

Explanation

The midpoint is (3k,5k)(3k,5k) and the gradient of ABAB is 44, so the bisector has gradient −14-\tfrac14: y−5k=−14(x−3k)y-5k=-\tfrac14(x-3k), i.e. x+4y=23kx+4y=23k.

Derived from ZIMSEC Mathematics Paper 1, June 2019, Q1

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