To complete the square for the quadratic equation \( x^2 - 6x + 6 = 0 \), we need to focus on the quadratic and linear terms, \( x^2 - 6x \).
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Identify the coefficient of the \( x \) term: The coefficient of \( x \) is \(-6\).
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Take half of this coefficient and square it:
- Half of \(-6\) is \(-3\).
- Squaring \(-3\) gives \((-3)^2 = 9\).
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Add this square (9) to both sides of the equation to keep it balanced.
Thus, the number that needs to be added to complete the square is \( 9 \).
The equation can be rewritten as follows after adding 9 to both sides:
\[ x^2 - 6x + 9 = 3 \]
Now, \( x^2 - 6x + 9 \) can be factored as \( (x - 3)^2 \).
In summary, the number that would have to be added to complete the square is \( 9 \).