Comparison

Comparing the Factorizations of z2+4z5z^2 + 4z - 5 and z24z5z^2 - 4z - 5

The trinomials z2+4z5z^2 + 4z - 5 and z24z5z^2 - 4z - 5 share the same leading term and constant term — only the sign of the middle coefficient differs (+4+4 versus 4-4). Despite this small change, the factored forms use the same pair of numbers (11 and 55) but with their signs swapped between the two binomials:

TrinomialFactors of 5-5 usedFactored form
z2+4z5z^2 + 4z - 51-1 and 55 (sum = 4)(z1)(z+5)(z - 1)(z + 5)
z24z5z^2 - 4z - 511 and 5-5 (sum = 4-4)(z+1)(z5)(z + 1)(z - 5)

The key observation is that when cc is negative, the same absolute-value factor pair can produce either a positive or a negative middle coefficient depending on which factor carries the negative sign. The factor with the larger absolute value determines the sign of the sum: here, 5>15 > 1, so assigning the positive sign to 55 gives a positive sum (+4+4), while assigning the negative sign to 55 gives a negative sum (4-4). This is why both sign arrangements must always be tested when cc is negative.

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Updated 2026-04-21

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