Example

Completing the Square for y29yy^2 - 9y

To complete the square for the expression y29yy^2 - 9y, identify the coefficient of the linear term, which is 9-9. Next, calculate the square of half of this coefficient. Taking half of an odd number produces a fraction: 12(9)=92\frac{1}{2} \cdot (-9) = -\frac{9}{2}. Squaring this fraction yields a positive constant: (92)2=814\left(-\frac{9}{2}\right)^2 = \frac{81}{4}. Adding this value to the expression forms the perfect square trinomial y29y+814y^2 - 9y + \frac{81}{4}. This trinomial can then be factored into the binomial squared (y92)2\left(y - \frac{9}{2}\right)^2. This example illustrates the process when the linear coefficient is a negative odd integer, which results in working with fractional values for both the constant and the binomial square.

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Updated 2026-05-15

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