WAEC 2022 · Paper 1 · Q13

If $5x + 3y = 4$ and $5x - 3y = 2$ , what is the value of $(25x^2 - 9y^2)$ ?

Worked solution (try it first)

$25x^2 - 9y^2$ is a difference of two squares: $(5x)^2 - (3y)^2 = (5x + 3y)(5x - 3y)$ .
Put in the two given values: $4 \times 2 = 8$ , option D.