What must be added to the expression x\(^2\) - 18x to make it a perfect square?

Answer Details

To make the expression x\(^2\) - 18x a perfect square, we need to add a constant term. The constant term should be half of the coefficient of x squared, which is (18/2)\(^2\) = 81. Therefore, we need to add 81 to the expression to make it a perfect square.
Adding 81 to the expression gives:
x\(^2\) - 18x + 81 = (x - 9)\(^2\)
Thus, the answer is 81.