## Elementary and Intermediate Algebra: Concepts & Applications (6th Edition)

$(a-4)(a-5)$
The 2 numbers whose product is $ac= 1(20)=20$ and whose sum is $b= -9$ are $\left\{ -4,-5 \right\}.$ Using these two numbers to decompose the middle term of the given trinomial, $a^2-9a+20 ,$ then the factored form is \begin{array}{l} a^2-4a-5a+20 \\\\= (a^2-4a)-(5a-20) \\\\= a(a-4)-5(a-4) \\\\= (a-4)(a-5) .\end{array}