To make the expression \( y^2 + 12y \) a perfect square, we need to complete the square.
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Start with the given expression: \[ y^2 + 12y \]
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To complete the square, we take the coefficient of \( y \) (which is 12), halve it, and then square it: \[ \left(\frac{12}{2}\right)^2 = 6^2 = 36 \]
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Add and subtract this value (36) inside the expression: \[ y^2 + 12y = (y^2 + 12y + 36) - 36 \]
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Now, rewrite the expression: \[ (y + 6)^2 - 36 \]
This shows that to make \( y^2 + 12y \) a perfect square, you need to add 36. The result is the perfect square \( (y + 6)^2 \).