Find the number to add to x^2-18x to make it a perfect square trinomial. Write that trinomial as the square of a binomial.
To find the number to add to x^2 - 18x to make it a perfect square trinomial, we need to take half of the coefficient of x (which is -18) and square it.
Half of -18 is -9, and -9 squared is 81.
Therefore, the number to add to x^2 - 18x to make it a perfect square trinomial is 81.
Now, we can write the perfect square trinomial as the square of a binomial:
(x - 9)^2 = x^2 - 18x + 81
So, the answer is (x - 9)^2.