Which value must be added to the expression x2 + 16x to make it a perfect-square trinomial?

To make x² + 16x a perfect-square trinomial, take half the coefficient of x and square it: (16 ÷ 2)² = 8² = 64. Then x² + 16x + 64 = (x + 8)², so the value is 64, choice C.

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