Example

Factoring q2−2q−15q^2 - 2q - 15

Factor q2−2q−15q^2 - 2q - 15 by applying the trinomial factoring strategy. The constant term −15-15 is negative, so the two numbers must have opposite signs — one positive and one negative. We need factors of −15-15 whose sum is −2-2.

Step 1 — Set up two binomials with first terms qq: (q)(q)(q\quad)(q\quad).

Step 2 — Find two numbers with opposite signs that multiply to −15-15 and add to −2-2. List all factor pairs of −15-15 and check their sums:

Factors of −15-15Sum of factors
1,−151, -151+(−15)=−141 + (-15) = -14
−1,15-1, 15−1+15=14-1 + 15 = 14
3,−53, -53+(−5)=−23 + (-5) = -2 ✓
−3,5-3, 5−3+5=2-3 + 5 = 2

The pair 33 and −5-5 has a product of −15-15 and a sum of −2-2.

Step 3 — Use 33 and −5-5 as the last terms of the binomials: (q+3)(q−5)(q + 3)(q - 5).

Step 4 — Check by multiplying: (q+3)(q−5)=q2−5q+3q−15=q2−2q−15(q + 3)(q - 5) = q^2 - 5q + 3q - 15 = q^2 - 2q - 15 ✓.

The factored form is (q+3)(q−5)(q + 3)(q - 5). This example demonstrates a case where cc is negative and the constant has multiple factor pairs to consider. All four sign arrangements of the factor pairs of −15-15 must be tested — only the pair 33 and −5-5 produces the required sum of −2-2.

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Updated 2026-04-21

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