## sam0004 one year ago Lim as x approaches pi/6 of (sin(x)-sin(pi/6))/x-pi/6

1. myininaya

compare to definition of derivative

2. myininaya

$\lim_{x \rightarrow a}\frac{f(x)-f(a)}{x-a}=f'(a)$

3. sam0004

So the limit would = 1?

4. myininaya

no

5. myininaya

what is f?

6. sam0004

...?

7. myininaya

what is f(x) and what is a?

8. sam0004

F(x)=sin(x), f(a)=sin(pi/6)

9. myininaya

so what is f'?

10. sam0004

Cos(x)-cos(pi/6)?

11. myininaya

:( if f=sin(x), then f'=?

12. sam0004

Cos(x)

13. myininaya

14. sam0004

A=sin(pi/6)?

15. myininaya

$\lim_{x \rightarrow a}\frac{f(x)-f(a)}{x-a}=f'(a) \text{ or we could say it equals } =(f(x))'|_{x=a}$

16. myininaya

$\lim_{x \rightarrow \frac{\pi}{6}}\frac{\sin(x)-\sin(\frac{\pi}{6})}{x-\frac{\pi}{6}}=(\sin(x))'|_{x=\frac{\pi}{6}}$

17. myininaya

do you see all i'm trying to get you to do is use the definition of derivative

18. myininaya

you are suppose to determine f and a then find f' then plug a into f' to get the result you are searching for

19. sam0004

Sqrt(3)/2?

20. myininaya

yep

21. sam0004

Thank you

22. sam0004

Any chance you can help with critical numbers?

23. myininaya

sure

24. sam0004

F(x)=4x/x^2+1

25. sam0004

I know i need the derivative

26. myininaya

do you mean 4x/(x^2+1)? Also what did you get for the derivative?

27. sam0004

Yes. F'(x)=(-4x^2+4)/(x^2+1)^2

28. myininaya

thats great now find when F'=0 Also we don't have to worry when F' DNE because x^2+1 is always positive. so F'=0 when top=0 so when -4x^2+4=0 solve for x now

29. sam0004

Sqrt(-1)

30. myininaya

I think you should retry solving -4x^2+4=0 I will give you a (you will get two real solutions)

31. sam0004

...+or-sqrt(-1)

32. sam0004

+or-sqrt(1)

33. myininaya

right sqrt(1)=1 so x=1 or x=-1

34. myininaya

those are the critical numbers for F.

35. sam0004

Wow...

36. sam0004

Find the intervals where f(x)=4/(x^+1) is increasing or decreasing.

37. myininaya

is that f(x)=4/(x+1)? like you weren't trying to write an exponent right?

38. sam0004

I Find the derivative...then set the denominator equal to zero...put those numbers on a number line and plug in numbers to find the intervals?

39. sam0004

I was writing an exponent

40. sam0004

I apologize for missing the exponent

41. sam0004

It should have said 4/(x^2+1)

42. myininaya

well I don't know what f you are looking at exactly. but yeah you definitely need to find f' because f' tells you if f is decreasing or increasing or neither f'=0 means the function is resting f'>0 means f is increasing f'<0 means f is decreasing I first find when f' =0 and when f' does not exist and then test each interval around those numbers to see if the function is increasing there or decreasing there

43. myininaya

so what is f' first?

44. sam0004

-8x/(x^4+1)

45. sam0004

X=-1 and x=1

46. myininaya

(x^2+1)^2 does not equal x^4+1

47. myininaya

just leave is at (x^2+1)^2

48. sam0004

Increasing from negative infinity to 0 and decreasing from 0 to positive infinity

49. myininaya

also x^2+1 is always positive so f' and f exist everywhere so you just need to find when f'=0 which is when -8x=0 which is when x=?

50. myininaya

ok yep

51. sam0004

F(x)=3x^2-12x=5 over the closed interval 0,3. Find absolute extrema.

52. sam0004

Do i again begin with taking the derivative?

53. myininaya

see if there any critical numbers between 0 and 3 first

54. myininaya

which means you need to find f' which all means you need find when f'=0 this looks like you meant to write a polynomial so we don't need to worry when f' dne

55. sam0004

X=2

56. myininaya

ok do F(0) and F(2) and F(3) which ever one of these are the lowest value is the absolute min value which ever one of these are the highest value is the absolute max value

57. sam0004

Ok so you use the tow numbers in the interval and the critical number?

58. myininaya

also I don't know what your function is exactly critical numbers in between the critical numbers and any included endpoints just plug them back into the original function

59. myininaya

you have two equal signs

60. myininaya

i'm guessing that second equal sign is a + or a -

61. myininaya

can't decide which one

62. sam0004

Should be +5

63. myininaya

ok do you want me to check your answers for this one or are you just going through problems and getting ideas on how to do them so you can pass your final

64. myininaya

like it seems like you are studying

65. sam0004

I just need ideas on how to pass my final...i struggle with a few areas.

66. myininaya

67. myininaya

I have to go do some house stuff good luck on your final

68. myininaya