Each of these expressions is a polynomial in two variables. The first expression is a perfect square trinomial, the second expression is a perfect square trinomial, and the third expression is a perfect square trinomial.

X^2 + 2xy + y^2 can be factored as (X+y)^2p^2 - 2pq + q^2 can be factored as (p-q)^2a^2 + 12a + 36 can be factored as (a+6)^2

These are all examples of perfect square trinomials, which are polynomials that can be factored into the square of a binomial.