1. ## Factorise help

$\displaystyle 4x^2+2x-20$

2. Originally Posted by lilly

$\displaystyle 4x^2+2x-20$

Well there are many different ways to do this, but here is one...

For any quadratic $\displaystyle ax^2+bx+c$ multiply a and c together to get

$\displaystyle a\cdot c =4\cdot (-20)=-80$

Now we need to find factors of the above that add up to b

$\displaystyle b=2$ and note that $\displaystyle -8 \cdot 10=-80 \mbox{ and }-8+10=2$

So now we rewrite the b term as the sum above i.e

$\displaystyle 2x=-8x+10x$ to get

$\displaystyle 4x^2-8x+10x-20$ now group the first and last two terms to get

$\displaystyle 4x^2-8x+10x-20 \iff (4x^2-8x)+(10x-20)$

Now factor each grouping

$\displaystyle 4x(x-2)+10(x-2) = (x-2)(4x+10)=2(x-2)(2x+5)$

3. Interesting... this is the same question as in
http://www.mathhelpforum.com/math-he...rise-help.html
And both of the original posters are from the same country. Coincidence?

01

4. Are you suggesting that a person would post the same question twice?

Surely not!