Final answer:
When solving the equation by completing the square, the perfect square trinomial is factored as (x + 10)².
Step-by-step explanation:
When solving the equation by completing the square, the perfect square trinomial is factored by taking the square root of the coefficient of x² and adding it to both sides of the equation. The trinomial (x + 10)² can be factored from x² + 20x + 100.
Therefore, the correct answer is (x + 10)².
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