WAEC 2016 · Paper 2 · Q2

(a)
Given that $(5, 2)$ , $(-4, k)$ and $(2, 1)$ lie on a straight line, find the value of $k$ .

Worked solution (try it first)

On one line, the gradients between pairs of points are equal.
From $(2, 1)$ to $(5, 2)$ : $\dfrac{2 - 1}{5 - 2} = \dfrac13$ .
From $(-4, k)$ to $(5, 2)$ : $\dfrac{2 - k}{5 + 4} = \dfrac13$ , so $2 - k = 3$ and $k = -1$ .