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

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