3q^2 + 16q + 5

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- Apr 11th 2010, 06:31 PMkenzie103109factor the equation
3q^2 + 16q + 5

- Apr 11th 2010, 06:38 PMmaddas
I look for a solution of the form (3q-a)(q-b). Expanding $\displaystyle 3q^2 - (3b+a)q +ab$. 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 $\displaystyle 3q^2 + 16q + 5= (3q+1)(q+5)$.

- Apr 11th 2010, 06:48 PMmag6
No......my previous answer is wrong......sorry about tht!!! (Sadsmile)

3q^2 + 16q +5

3q^2 +15q +q +5

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

(3q+1) (q+5) - Apr 11th 2010, 06:48 PMCaptainBlack
$\displaystyle P(q)=3q^2 + 16q + 5$

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

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

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

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

CB