Answer:
"Perfect square trinomials" are quadratics which are the results of squaring binomials. (Remember that "trinomial" means "three-term polynomial".)
For instance:
(x + 3)2
= (x + 3)(x + 3)
= x2 + 6x + 9
...so x2 + 6x + 9 is a perfect square trinomial.
