To determine the value of \( c \) that will make the left-hand side of the equation \( x^2 - 8x + c \) a perfect square trinomial, we can use the formula for completing the square.
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Start with the quadratic \( x^2 - 8x \).
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Take the coefficient of \( x \) (which is \(-8\)), divide it by 2, and square it:
\[ \left( \frac{-8}{2} \right)^2 = (-4)^2 = 16 \]
So, the value of \( c \) that makes \( x^2 - 8x + c \) a perfect square trinomial is \( 16 \).
The complete expression becomes:
\[ x^2 - 8x + 16 = (x - 4)^2 \]
Thus, the answer is:
16