Factories:

(l + m)² - (l –m)²

We have,

(l + m)² - (l –m)²

= {(l + m) + (l – m)}{(l + m) – (l – m)}

By using the formula a² – b² = (a + b)(a – b)

We get,

(l + m)² - (l –m)² = {(l + m) + (l – m)}{(l + m) – (l – m)}

= (l + m + l – m)(l + m – l + m)

= (2l)(2m)

= 4lm