A perfect square trinomial is a trinomial that can be written as the square of a binomial. There are two general types of perfect square trinomials that can be useful when dealing with quadratic functions or quadratic equations.

Perfect Square Trinomial	Square of Binomial
$a^2+2ab+b^2$	$(a+b{)}^2$
$a^2-2ab+b^2$	$(a-b{)}^2$

A perfect square trinomial should have one of the forms shown in the table — that is, the first and the last terms are perfect squares and the middle term is two times the product of the square roots of the first and last terms. Once a quadratic expression is recognized as a perfect square trinomial, it can be written as a square of a binomial by factoring the perfect square trinomial.