Dimitri is solving the equation x2 – 10x = 21. Which value must be added to both sides of the equation to make the left side a perfect-square trinomial?

To make x² − 10x a perfect-square trinomial, take half of the coefficient of x and square it: (−10 ÷ 2)² = 25. Adding 25 to both sides makes the left side x² − 10x + 25 = (x − 5)². Therefore, the value is 25.

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