## mathcalculus 2 years ago mathcalculus HELP: Determine the values of where the tangent line is horizontal for the function:

1. mathcalculus

2. mathcalculus

i know i have to find the derivative and set it to zero but it's not coming out to be correct. can anyone help me

3. mathcalculus

so i did 3x^2-6x-4=0

4. mathcalculus

i tried to use the quadratic formula and got sqrt(21)+3/3

5. mathcalculus

+ or - sqrt(21)+3/3

6. mathcalculus

also aka 2.52753...

7. dumbcow

derivative seems correct

8. mathcalculus

to find what x = to... ?

9. mathcalculus

10. mathcalculus

@dumbcow dont i have to do that to find x??

11. mathcalculus

im getting it wrong,

12. dumbcow

yes everything looks correct to me...do they want exact or decimal answer?

13. dumbcow
14. mathcalculus

can i use quadratic formula for ths one?

15. mathcalculus

@dumbcow

16. mathcalculus

??

17. mathcalculus

????????????????????????

18. dumbcow

yes of course you use quadratic formula...thats how you got your answer that i said looked correct

19. mathcalculus

im not getting that using quadratic formula

20. mathcalculus

im getting something totally different and wrong

21. dumbcow

$x = \frac{3 \pm \sqrt{21}}{3}$

22. mathcalculus

yes! but from 6+2*sqrt(21)/6?

23. mathcalculus

shouldnt it be 1+ or - sqrt 21/3

24. dumbcow

same thing, just a matter of splitting up fraction or keeping it as single fraction

25. mathcalculus

no it's not because the answer is 1+ sqrt (7/3) gives me 2.52753 and 1+sqrt 21/3 gives me 1.86086!

26. mathcalculus

@dumbcow

27. dumbcow

hmm $\frac{\sqrt{7}}{\sqrt{3}} = \frac{\sqrt{21}}{3}$

28. mathcalculus

hmmm?

29. mathcalculus

@dumbcow

30. mathcalculus

try it on a calculator.

31. dumbcow
32. dumbcow

oh you added the "1" before dividing by "3"

33. mathcalculus

okay from my answer.... not the internet.. 6+2 sort(21)/3.... where do i go from there?

34. dumbcow

well it should be a 6 on bottom

35. mathcalculus

36. mathcalculus

okay sure a 6. i thought it could be a 3 to narrow it down

37. mathcalculus

so 6+or-2sqrt(21)/6

38. dumbcow

i dont see the problem, you posted answer in beginning... " i tried to use the quadratic formula and got sqrt(21)+3/3 "

39. mathcalculus

so it is a 3 in the denominator!

40. dumbcow

$\frac{6 \pm 2\sqrt{21}}{6} = \frac{2(3 \pm \sqrt{21})}{6} = \frac{3 \pm \sqrt{21}}{3}$