JAMB 2017 · UTME · Q3

Find the value of $a$ if the line $2y - ax + 4 = 0$ is perpendicular to the line $y + \frac14x - 1 = 0$ .

Worked solution (try it first)

Rearrange $2y - ax + 4 = 0$ : $2y = ax - 4$ , so the gradient is $\frac a2$ .
Rearrange $y + \frac14x - 1 = 0$ : $y = -\frac14x + 1$ , so the gradient is $-\frac14$ .
Perpendicular gradients multiply to $-1$ : $\frac a2 \times \left(-\frac14\right) = -1$ , so $\frac a8 = 1$ .
So $a = 8$ , option C.