Example

Factoring z24z5z^2 - 4z - 5

Factor z24z5z^2 - 4z - 5 by applying the trinomial factoring strategy. The constant term 5-5 is negative, so the two numbers must have opposite signs. This time, we need factors of 5-5 whose sum is 4-4.

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

Step 2 — Find two numbers with opposite signs that multiply to 5-5 and add to 4-4. List both sign arrangements:

Factors of 5-5Sum of factors
1,51, -51+(5)=41 + (-5) = -4
1,5-1, 51+5=4-1 + 5 = 4

The pair 11 and 5-5 has a product of 5-5 and a sum of 4-4.

Step 3 — Use 11 and 5-5 as the last terms of the binomials: (z+1)(z5)(z + 1)(z - 5).

Step 4 — Check by multiplying: (z+1)(z5)=z25z+z5=z24z5(z + 1)(z - 5) = z^2 - 5z + z - 5 = z^2 - 4z - 5 ✓.

The factored form is (z+1)(z5)(z + 1)(z - 5). Compared to z2+4z5=(z1)(z+5)z^2 + 4z - 5 = (z - 1)(z + 5), the same factor pair of 5-5 is used, but with the signs assigned to opposite binomials. This shows that it is critical to select the sign arrangement that produces the correct sign of the middle term.

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Updated 2026-04-21

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