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How to Complete a Square

To complete the square of an expression of the form x2+bxx^2 + bx, follow a three-step procedure:

Step 1. Identify bb, the coefficient of the linear term xx.

Step 2. Compute (12b)2\left(\frac{1}{2}b\right)^2 — take half of bb and square the result. This value is the constant needed to complete the square.

Step 3. Add (12b)2\left(\frac{1}{2}b\right)^2 to x2+bxx^2 + bx. The resulting expression x2+bx+(12b)2x^2 + bx + \left(\frac{1}{2}b\right)^2 is a perfect square trinomial that factors as (x+12b)2\left(x + \frac{1}{2}b\right)^2.

When bb is negative, the procedure works the same way — halving a negative number produces a negative result, but squaring it always yields a positive constant. The factored binomial square then uses subtraction rather than addition.

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Updated 2026-04-21

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