Concept

Factoring x2+bx+cx^2 + bx + c When cc Is Negative

When factoring a trinomial of the form x2+bx+cx^2 + bx + c where the constant term cc is negative, the two numbers mm and nn in the factored form (x+m)(x+n)(x + m)(x + n) must have different signs — one positive and one negative.

This follows from the connection between factoring and FOIL multiplication. In the expansion (x+m)(x+n)=x2+(m+n)x+mn(x + m)(x + n) = x^2 + (m + n)x + mn, the last term c=mnc = mn is the product of the last terms of the two binomials. A negative product can only result from multiplying two numbers with opposite signs — one positive and one negative. Therefore, when cc is negative, exactly one of mm and nn must be positive and the other must be negative.

Extra care is needed when choosing which factor is positive and which is negative, because this choice also determines the sign of the middle coefficient b=m+nb = m + n. Since mm and nn have different signs, their sum could be positive or negative depending on which factor has the larger absolute value. Both sign arrangements of a factor pair may need to be tested to ensure the middle term comes out correctly.

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Updated 2026-04-30

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