# factor the equation

• Apr 11th 2010, 06:31 PM
kenzie103109
factor the equation
3q^2 + 16q + 5
• Apr 11th 2010, 06:38 PM
I look for a solution of the form (3q-a)(q-b). Expanding $3q^2 - (3b+a)q +ab$. Equating the coefficients of like terms gives ab=5 and 3b+a=-16. Thinking for a moment will convince you that b=-5 and a=-1 satisfy these equations. So $3q^2 + 16q + 5= (3q+1)(q+5)$.
• Apr 11th 2010, 06:48 PM
mag6

3q^2 + 16q +5

3q^2 +15q +q +5

3q( q+5) +1 (q+5)

(3q+1) (q+5)
• Apr 11th 2010, 06:48 PM
CaptainBlack
Quote:

Originally Posted by kenzie103109
3q^2 + 16q + 5

$P(q)=3q^2 + 16q + 5$

The rational roots theorem suggests that $\pm 5$, $\pm 1/3$ and $\pm 5/3$ are the only possible rational roots for this quadratic in $q$. Descartes rule of signs tells you it has no positive roots, so that reduces the list to $-5, -1/3$ and $-5/3$. Now trying these out shows that $P(-5)=0$ and $P(-1/3)=0$, so we have:

$P(q)=k(q+1/3)(q+5)$

and to match the leading coefficient of $P(q)$ we must have $k=3$ so:

$P(q)=(3q+1)(q+5)$

CB