WAEC 2020 · Paper 1 · Q14

Make $m$ the subject of the relation $k = \sqrt{\dfrac{m - y}{m + 1}}$ .

Worked solution (try it first)

Square both sides and multiply by $m + 1$ : $k^2m + k^2 = m - y$ .
Collect the $m$ terms on the right: $k^2 + y = m - k^2m$ , so $k^2 + y = m(1 - k^2)$ .
Divide by $1 - k^2$ : $m = \dfrac{y + k^2}{1 - k^2}$ , option B.