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Trinomials - Undoing FOIL

Here's one last weird thing on these:

x^2 - 5xy - 36y^2 ... the y^2 term is different!

Since we have a back variable, we just start with

( x y ) ( x y )

Our possibilities for the 36 are

1 * 36 and 2 * 18 and 3 * 12 and 4 * 9 and 6 * 6

Which pair will get us a -5 ?

4 and 9 ... ( 4 ) ( -9 ) = -36 and 4 - 9 = -5

 

So,    x^2 - 5xy - 36y^2 = ( x + 4y ) ( x - 9y )

FOIL this so you'll believe the y thing!

YOUR TURN:

Factor  a^2 - 8ab - 48b^2