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How to Solve a Quadratic Equation by Completing the Square

To solve a quadratic equation of the form x2+bx+c=0x^2 + bx + c = 0 by completing the square, follow a six-step procedure:

Step 1. Isolate the variable terms on one side and the constant terms on the other. Move the constant cc to the right side of the equation so the left side contains only x2+bxx^2 + bx.

Step 2. Find (12b)2\left(\frac{1}{2} \cdot b\right)^2, the number needed to complete the square, and add it to both sides of the equation. Take half the coefficient of the linear term, square it, and add that value to both sides. This preserves the equality — whatever is added to one side must also be added to the other.

Step 3. Factor the perfect square trinomial as a binomial square. The left side is now a perfect square trinomial that factors into the square of a binomial.

Step 4. Use the Square Root Property. Apply x+b2=±kx + \frac{b}{2} = \pm\sqrt{k}, where kk is the simplified value on the right side.

Step 5. Simplify the radical and solve the two resulting equations. If the radicand is a perfect square, simplify to an integer; otherwise, simplify using the Product Property of Square Roots.

Step 6. Check the solutions. Substitute each value back into the original equation to confirm both sides are equal.

This procedure combines the completing-the-square technique (Steps 1–3) with the Square Root Property (Steps 4–5) to solve quadratic equations that may not be factorable. The critical requirement in Step 2 is that the same constant must be added to both sides of the equation to maintain the equality.

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Updated 2026-04-21

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Ch.10 Quadratic Equations - Elementary Algebra @ OpenStax

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