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Trinomials - Undoing FOIL

YOUR TURN:

Factor  x^2 - 20x - 21

This one's harder:

Factor   b^2 - 7b + 12

This one has more possibilities for the 12:

1 * 12 and 2 * 6 and 3 * 4

Which pair will score us a -7 for the middle guy?

3 and 4 ... ( -3 ) ( -4 ) = 12 and -3 - 4 = -7
 

So,    b^2 - 7b + 12 = ( b - 3 ) ( b - 4 )

 

TRY IT:

Factory^2 + 12y - 28