Example

Factoring z2−4z−5z^2 - 4z - 5

Factor z2−4z−5z^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, 5−1+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)(z−5)(z + 1)(z - 5).

Step 4 — Check by multiplying: (z+1)(z−5)=z2−5z+z−5=z2−4z−5(z + 1)(z - 5) = z^2 - 5z + z - 5 = z^2 - 4z - 5 ✓.

The factored form is (z+1)(z−5)(z + 1)(z - 5). Compared to z2+4z−5=(z−1)(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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