## savvers Group Title -4sqrt(x+9)=20 can anyone help? one year ago one year ago

1. geoffb Group Title

$-4 \sqrt{(x+9)} = 20$ Like this? At what part are you getting stuck?

2. savvers Group Title

well i get x=16 but when i try to see if its true i get -20=20

3. savvers Group Title

and yes like that

4. geoffb Group Title

Yeah, with that -4 outside the square root, I think it's undefined.

5. savvers Group Title

yeah your probably right. extraneous solution?

6. geoffb Group Title

If we defined $$y$$ as $$\sqrt{(x+9)}$$, we would get $$-4y = 20$$, with $$y = -5$$, and there is no way $$\sqrt{(x+9)} = -5$$

7. savvers Group Title

hmmm i dont know. but when we divide -4 we get $\sqrt{x+9}=-5$

8. geoffb Group Title

$$\large{\sqrt{-x} = i \sqrt{x}}$$ right?

9. savvers Group Title

yeah

10. geoffb Group Title

Okay, can we use that somehow?

11. geoffb Group Title

Oh wait... We can square both sides, right?

12. savvers Group Title

hmmm yeah

13. savvers Group Title

and we get x+9=25 because -5x-5=25

14. geoffb Group Title

Right, so you were right with 16; it is an extraneous root.

15. savvers Group Title

yeah it looks like that then and thanks man

16. geoffb Group Title

Hey, no problem. :)