An expression of the form a^2-2ab+b^2 can be factored to a perfect square. a^2-2ab+b^2=(a-b)^2 An example is the expression x^2-10x+25 which can be written as x^2-2* x * 5+5^2, and thus, be factored to the square of the binomial x-5. x^2-2* x * 5+5^2=(x-5)^2