Question

Find all values of k so that the trinomial x^2 + kx - 35 can be factored using integers

Answer 1

$(x-r_1)(x-r_2)=x^2-(r_1+r_2)x+r_1r_2$

So the trinomial $x^2+kx-35$ can be factored as long as

$\begin{cases}r_1+r_2=-k\\r_1r_2=-35\end{cases}$

has integer solutions for $r_1,r_2$. Clearly, both have to be factors of -35, which leaves only a handful of cases:

$(r_1,r_2)=(1,-35)\implies r_1+r_2=-34$
$(r_1,r_2)=(-1,35)\implies r_1+r_2=34$
$(r_1,r_2)=(5,-7)\implies r_1+r_2=-2$
$(r_1,r_2)=(-5,7)\implies r_1+r_2=2$

So the possible values of $k$ are $\pm34$ and $\pm2$.