a•c  method for trinomials:

General form:       ax²+bx+c=0

1)  Multiply  a • c                    

2) Find the 2 factors (divide evenly) of ac that will combine (add/subtract) to make “b”.

→ Signs:

→ If ac is positive, then both the new factors are the same sign as “b”

→ If ac is negative, then the signs are different, and the larger factor

     Is the same sign as “b”.                                                            .  

→ List factors of ac until you find the right combination.

3) Rewrite the equation replacing bx with the 2 new factors → remember to put in the letters!

4) Look for the GCF (greatest common factor) in the first two terms,

          → bring down the sign of the third term

          →look for the GCF in the second two terms

5) The parentheses must match!

6) Bring down the common parentheses; put the coefficient in a second parenthesis.

              (-1)

Ex:                                                          2•-15=-30     ←ac          

    2x² + 5x   -  6x -15 = 0                                                                  +1  -30

                    ↓

     x(2x +5)  -  3(2x +5) =0                                                                +2  -15