Final answer:
To make the expression x² +8x a perfect square trinomial, you would add 16.
Step-by-step explanation:
To make the expression x² +8x a perfect square trinomial, we need to find a number that, when added to the expression, will result in a square trinomial. The number we need to add is half the coefficient of the middle term squared. In this case, half of 8 is 4, and 4 squared is 16. Therefore, we would add 16 to the expression to make it a perfect square trinomial.
Learn more about Perfect square trinomials