Answer: The expression X^2 + _x + 100 is the square of a binomial if the missing number is -b/2 and b^2 - 4ac = 0.
Step-by-step explanation: The general form of the square of a binomial is (x + b)^2 = x^2 + 2bx + b^2.
In this case, the missing number is -b/2, and the expression becomes X^2 - x + 100.
Now to find the value of x, we can use the equation b^2 - 4ac = 0
The equation becomes x^2 - 41100 = 0
Which gives x^2 - 400 = 0
So x = ± 20.
So the possible values for the missing number that make the expression the square of a binomial is -20/2 = -10 and 10.