Activity (Process)

Factoring Trinomials of the Form ax2+bx+cax^2 + bx + c Using the ac Method

The "ac" method (also called the grouping method) is a systematic alternative to trial and error for factoring trinomials of the form ax2+bx+cax^2 + bx + c. It extends the approach used for trinomials with a leading coefficient of 1 — where two numbers whose product and sum match the constant and middle coefficient are sought — by incorporating the product aca \cdot c and then applying factoring by grouping. Because the method follows a fixed sequence of steps, it is very structured and always works. The procedure has six steps:

  1. Factor any GCF from all three terms before proceeding.
  2. Find the product aca \cdot c (the coefficient of the squared term multiplied by the constant term).
  3. Find two numbers mm and nn such that mn=acm \cdot n = a \cdot c and m+n=bm + n = b.
  4. Split the middle term bxbx into two terms using mm and nn: rewrite ax2+bx+cax^2 + bx + c as ax2+mx+nx+cax^2 + mx + nx + c. This converts the three-term trinomial into a four-term polynomial.
  5. Factor by grouping. Group the four terms into two pairs and factor the GCF from each pair, then extract the common binomial factor.
  6. Check by multiplying the factors to verify they produce the original trinomial.

The key insight is that the numbers mm and nn whose product equals acac and whose sum equals bb provide exactly the right decomposition of the middle term so that factoring by grouping succeeds. When the constant term cc is negative, the product acac will typically be negative as well, so the two numbers mm and nn must have opposite signs — one positive and one negative.

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Updated 2026-04-30

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