Concept

Completing the Square When the Leading Coefficient Is Not One

The completing-the-square procedure requires the coefficient of the x2x^2 term to be 11, so that the left side of the equation has the form x2+bx+cx^2 + bx + c. When a quadratic equation ax2+bx+c=0ax^2 + bx + c = 0 has a leading coefficient a1a \neq 1, preliminary steps must be taken to reduce that coefficient to 11 before the standard six-step completing-the-square procedure can begin.

There are two strategies for achieving a leading coefficient of 11:

Strategy 1 — Factor out the leading coefficient as a GCF. When the leading coefficient divides evenly into all three terms of the trinomial, factor it out as the greatest common factor, then divide both sides of the equation by that coefficient. For example, in 3x212x15=03x^2 - 12x - 15 = 0, the coefficient 33 divides into 33, 1212, and 1515, so factoring gives 3(x24x5)=03(x^2 - 4x - 5) = 0. Dividing both sides by 33 produces x24x5=0x^2 - 4x - 5 = 0, which now has a leading coefficient of 11.

Strategy 2 — Divide both sides by the leading coefficient. When the leading coefficient does not divide evenly into all terms, divide every term on both sides by the leading coefficient. This produces fraction coefficients for the linear and/or constant terms. The completing-the-square procedure then continues with those fractions, using the same technique already practiced for expressions with fractional linear coefficients.

In both strategies, the goal is the same: transform the equation so that the x2x^2 term stands alone with a coefficient of 11, enabling the standard completing-the-square steps to proceed.

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Updated 2026-04-21

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