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How do you determine whether to use plus or minus signs in the binomial factors of a trinomial of the form x^2 + bx + c where b and c may be positive or negative numbers?

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The rule of signs in binomial factors given a trinomial in the form :


x^2\pm bx\pm c

Note that b and c can be positive or negative.

And the factor may be :


(x\pm m)(x\pm n)

First thing to do is to check the sign of c.

If it is positive, the signs of m and n can be both positive or both negative.

Since positive multiplied by positive is positive and

negative multiplied by negative is also positive.


\begin{gathered} (+)*(+)=+ \\ (-)*(-)=+ \end{gathered}

If the sign of c is negative, one of the factors must be negative and the other must be positive, because negative multiplied by positive is negative.


(-)*(+)=-

After considering the sign of c, next to check is the sign of b.

Case 1 :

If b and c are both positive, we know that the factors can be both negative or both positive.

And since the sign of b is positive, we must follow the sign of it which is positive.

So both m and n are positive

For example :


x^2+5x+6

The factor will be :


(x+2)(x+3)

which are both positive.

Case 2 :

If b is negative and c is positive, we will follow the sign of b. m and n will be both negative.

For example :


x^2-5x+6

The factors are :


(x-2)(x-3)

which are both negative

Case 3 :

Now the third case is tricky.

If c is negative, we know that one of m and n is negative and the other one is positive.

The sign of the larger number between m and n will follow the sign of b.

For example :


x^2-x-6

c is negative, so one factor must be negative and the other must be positive. The sign of b is negative, so the larger factor must be negative also.

The factor will be :


(x-3)(x+2)

3 is larger than 2, so the negative sign of b will proceed to the factor 3.

User Bernhard Kausler
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