Math CentralQuandaries & Queries


Question from RAY, a student:
how u do the foil ?
the question (x+4)(x+5)


Don't "do the foil", use the distributive law, it is way cooler.

How you do that: multiply each term in the first bracket by the whole second bracket and add the results up. Then multiply everything in each second bracket by the stuff behind it. Then combine terms with the same x's in.

= x(x+5) + 4(x+5)
= x*x + x*5 + 4*x + 4*5
= x2 + 9x + 20

FOIL is a sort of "distributive law with welded-on training wheels". It gives the right answers to very simple problems but then you can't go any further. For instance, with FOIL you can't do (x2+2x+3)(x+1) and with the distributive law you can.

= x2(x+1) + 2x(x+1) + 3(x+1)
= x2*x + x2*1 + 2x*x + 2x*1 + 3*x + 3*1
= x3 + x2 + 2x2 + 2x + 3x + 3
= x3 + 3x2 + 5x + 3

Good Hunting!


About Math Central


Math Central is supported by the University of Regina and The Pacific Institute for the Mathematical Sciences.
Quandaries & Queries page Home page University of Regina PIMS