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## pablobegins 3 years ago Really annoying trigonometric graphing, help!

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1. pablobegins

2. pablobegins

I need 1,2,5, and 10-24

3. pablobegins

I'm just not grasping the concept.

4. amistre64

f(x) = tan πx/4 g(x) =1/2 sec πx/4 Approximate the interval where f < g. lets set this up then, we know f and g tan (πx/4) < 1/2 sec(πx/4)

5. amistre64

one thing that might help is to rewrite sec, or maybe even tan. into equivalent terms

6. pablobegins

how so?

7. amistre64

well, tan = sin/cos, right? and sec = 1/cos .... that might help us see the resemblence

8. amistre64

$\frac{sin (πx/4)}{cos (πx/4)} < \frac{1}{2}\frac{1}{cos(πx/4)}$ we can get rid of that 1/2 by multiplying by 2 $\frac{(2)sin (πx/4)}{cos (πx/4)} < \frac{1(2)}{2}\frac{1}{cos(πx/4)}$ $\frac{2sin (πx/4)}{cos (πx/4)} < \frac{1}{cos(πx/4)}$ now it looks better

9. amistre64

since the denominators are the same, lets equate numberators

10. amistre64

2sin(pi x/4) < 1

11. pablobegins

so now we graph the equation to find the point?

12. amistre64

id stick to the analysis rather than a graph; the graph can be used to dbl chk the results; but i doubt it will give a definitive result

13. amistre64

sin (t) < 1/2; well, when does sin(t) = 1/2?

14. pablobegins

how would we go about the analysis?

15. amistre64

divide each side by 2 to get: sin(t) < 1/2 im using "y" to help clean up the argument

16. amistre64

"t" that is ... cant type ;)

17. pablobegins

that's not one of the choices though :/ ,

18. amistre64

why would it be? we are only in the middle of it

19. GT

Hang in there and understand the steps.

20. pablobegins

OH! Gotcha. I just have a terrible teacher who goes about everything explaining really fast, and then expecting us to know all this. I'm sorry.

21. amistre64

it would be good to remember the basic angles of trig; i believe this is one of them

22. amistre64

|dw:1323627720794:dw|

23. amistre64

sin(t) = 1/2, when t = 30 degrees, or pi/6

24. pablobegins

is that the special right triangle?

25. amistre64

it is

26. amistre64

i have to remember the interval: -pi/4 to pi/4, that is between 45 and -45 degrees if i recall it correctly

27. amistre64

now i would see about equating pi x/4 and t pi x/4 = pi/6 divide off the pis x/4 = 1/6 and multiply off the 1/4 x = 4/6 , and simplify x = 2/3 so, if i did it right, it should be (-1, 2/3) but if you have questions as to why, it would be good to ask. The numbers tend to be unimportant and it is the process that matters.

28. pablobegins

why the -1?

29. amistre64

its part of our initial interval; and sin(pi *-1/4) is less than 1/2

30. pablobegins

i see. so for number 2 i would just multiply our original interval?

31. amistre64

it would be nice if we could, but no. trig functions are periodic, they do not act like linear functions. so its best to retrace the steps to make sure

32. pablobegins

or would it not change?

33. amistre64

2f just changes how high or low the graph would go; so id guess that its the same intervals. But i would still have to dbl chk the results

34. pablobegins

its not it.

35. pablobegins

like, number 1 was right, but 2f changed it in some way, because now it's not the same

36. amistre64

$2*\frac{sin (πx/4)}{cos (πx/4)} < 2*\frac{1}{2}\frac{1}{cos(πx/4)}$ $\frac{2sin (πx/4)}{cos (πx/4)} < \frac{1}{cos(πx/4)}$ $2sin (πx/4) < 1$ $sin (πx/4) < 1/2$ it looks to be the same

37. pablobegins

must be a problem with the software. now for number 5,i cant tell the graphs apart

38. amistre64

which one is number 5? this thing is hard to follow

39. pablobegins

graph for y=csc (x)

40. amistre64

well, I know csc is the humpbacks of sine|dw:1323628829369:dw|

41. amistre64

the closest one I see is A

42. amistre64

we can draw the sin(x) in A and see that csc rides the humpbacks

43. pablobegins

yeah, me too! thanks so much

44. pablobegins

number 10?

45. amistre64

id ditch the graph... see that sqrt(3)? that comes about in the 30-60-90 triangle

46. amistre64

and since cot = cos/sin; this is negative when either cos is negative OR sin is negative; but not both. that occurs in q2 and q4

47. pablobegins

so we would relate the degrees with a sin fuction?

48. amistre64

|dw:1323629102467:dw|

49. amistre64

almost; lets draw the 30-60-90 tri again and see what cot comes from

50. amistre64

|dw:1323629160526:dw| the basic angle is then cot(30) do you agree? 30 = pi/6

51. pablobegins

yeah, i see where you're coming from

52. amistre64

q2: 180-30 = 150 degrees q4: 360 - 30 = 330 degrees

53. amistre64

hmmm, 5pi/6 and 11pi/6 seem to fit in there somewhere. but i do get these mixed aroung at times :)

54. amistre64

i see the interval now; it one full revolution forward and one full revolution backwards

55. pablobegins

ignore that

56. pablobegins

it sent a previous message

57. pablobegins

would the pi/6 be negative? since were dealing with cot?

58. amistre64

|dw:1323629494595:dw|

59. amistre64

-pi/6, 5pi/6, -7pi/6, 11pi/6 is my best assessment

60. amistre64

thats option 4 right?

61. pablobegins

for 11, is the period 16/

62. pablobegins

YES, YOU WERE RIGHT ONCE AGAIN! :)

63. amistre64

tan and cot have a normal period of pi; this speeds that period up; by a factor of 8.; so id say its a period of 1/8

64. amistre64

that doesnt work out right ....

65. pablobegins

if we divide it by 2pi, you get 16

66. amistre64

that does sound familiar :)

67. amistre64

but the 2pi is wrong

68. amistre64

divide by pi; since this is a pi period 2pi is for sin and cos

69. pablobegins

so it would be 1/8?

70. amistre64

pi/(pi/8) = pi * 8/pi = 8

71. amistre64

normal period/w

72. pablobegins

and the normal period is 8??

73. amistre64

look again

74. amistre64

tan/cot has a normal pf pi everything else is 2pi

75. amistre64

pi/(w) = periodicity

76. amistre64

since w = pi/8 .... we plug it in

77. pablobegins

we end up with 1/8.

78. amistre64

$\frac{pi}{pi/8}=\frac{pi*8}{pi}=8$

79. pablobegins

oh, i was solving it incorrectly. can you explain 12 to me please? i know it's similar to 10, but i want to grasp the concept.

80. amistre64

since csc rides the humpbacks of sin; lets equate this to an angle of sin. csc = 1/sin is a good thing to remember

81. amistre64

$csc(x)=\frac{2\sqrt{3}}{3}$ $\frac{1}{sin(x)}=\frac{2\sqrt{3}}{3}$ $\frac{sin(x)}{1}=\frac{3}{2\sqrt{3}}$ and draw a triangle to help visualise it

82. amistre64

|dw:1323630627950:dw|

83. amistre64

this doesnt seem to match any of our basic triangles does it?

84. amistre64

maybe .... divide off sqrt(3) 2sqrt(3)/sqrt(3) = 2 3/sqrt(3) = 3sqrt(3)/3 = sqrt(3) its our 30-60-90 in disguise

85. amistre64

|dw:1323630843700:dw|

86. pablobegins

'disguise' lol

87. pablobegins

so we're just rearranging the equation so the triangle makes sense, in a way

88. amistre64

$csc(60) = \frac{2}{\sqrt{3}}$ $csc(60) = (\frac{2}{\sqrt{3}}*\frac{\sqrt{3}}{\sqrt{3}})$ $csc(60) = \frac{2\sqrt{3}}{3}$ so we are good :)

89. amistre64

60 = pi/3 .... so we need to adapt that to the interval

90. amistre64

|dw:1323631158475:dw|

91. amistre64

|dw:1323631248664:dw|

92. amistre64

thatss what, option 7?

93. pablobegins

seven is incorrect :-(nt and so is 3

94. amistre64

trig takes alot of mental gymnastics :)

95. amistre64

hmmm

96. amistre64

i think I know where I went astray at

97. amistre64

csc is only positive in q1 and q2, i confused it with a tan ....

98. amistre64

|dw:1323631597639:dw|

99. amistre64

-5,-4,1,2 ---------, might be better 3

100. pablobegins

what choice is that?

101. amistre64

option 4, since that aint in the choices

102. amistre64

youre right tho, these ARE really annoying ;)

103. pablobegins

finally correct! this thing penalizes me for every wrong answer too... now for the period of number 13, is 7?

104. amistre64

the value attached to x is out "w" and sec is a 2pi normal $\frac{2pi}{-7pi/2}$ $\frac{2pi*2}{-7pi}$ $\frac{2*2}{-7}=-4/7$ id go with option 1

105. pablobegins

ow do we find the frequency?

106. amistre64

frequency is how fast it moves along .... id have to look it up :)

107. amistre64
108. amistre64

freq = 1/period

109. pablobegins

initial phase?

110. amistre64

i dont recall ever doing an initial phase ...

111. amistre64

is that a phase shift perhaps?

112. pablobegins

it says initial phase, it might be referring to the same thing

113. amistre64

i cant make any sense of the online stuff. I doubt that it is the same thing then

114. amistre64

initial phase is when t=0 ....

115. amistre64

but i got no idea at the moment what the means

116. pablobegins

what about the period of 1 7? i got 10/9, but that's apparently wrong

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