## Elementary Algebra

$x^2+8x-9$
Using $(a+b)(c+d)=ac+(ad+bc)+bd$, or the FOIL Method, the given expression, $(x-1)(x+9) ,$ is equivalent to \begin{array}{l}\require{cancel} x^2+(9x-x)-9 \\\\= x^2+8x-9 .\end{array}