Question

Factorise
$(l+m{)}^2-(l-m{)}^2$

Answer 1

${(l+m)}^2-{(l-m)}^2$ is of the form $a^2-b^2=(a+b)(a-b)$

$=(l+m+l-m)(l+m-l+m)$

$=2l\times 2l$

$=4lm$