The formula for expanding the square of a binomial expression (a + b)^2 is:
(a + b)^2 = a^2 + 2ab + b^2
In this formula, the expression (a + b) is squared by multiplying it by itself. The resulting expanded form consists of three terms: the square of the first term (a^2), twice the product of the two terms (2ab), and the square of the second term (b^2).