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 in the factor pair must have opposite signs — one positive and one negative. We need factors of −5-5 whose sum is positive 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. List both sign arrangements of each factor pair of −5-5 and check their sums:

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

Both arrangements must be tested because the sign assignment determines whether the sum is positive or negative. The pair −1-1 and 55 has a product of −5-5 and a sum of 4.

Step 3 — Use −1-1 and 55 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). This example shows that when cc is negative, both sign arrangements of a factor pair must be considered to ensure the middle term has the correct sign.

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Updated 2026-04-21

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