## anonymous 5 years ago 0=-4t^2+20t+1 i need to figure out wat t is

1. anonymous

Have you learned the quadratic formula?

2. anonymous

yea but dat aint working it would be a=4 b=-20 c=1 can u try

3. anonymous

a = -4 b = 20 c = 1

4. anonymous

yea

5. anonymous

im trying 2 figure dis out

6. anonymous

(h(t)=-16t^2+vt+c where h is the approximate height in ft t is the time in motion in seconds v is the initial upward velocity in ft per second and c is the initial height in ft) A baseball player hits a pitched ball when it is 4ft above the ground the initial upward velocity is 80ft/s how long will it take for the ball to hit the ground???

7. anonymous

im stuck on the step with the quadratic formula

8. anonymous

So what is the initial velocity?

9. anonymous

Plug that in for v. What is the height above the ground? Plug that in for c.

10. anonymous

v is 80

11. anonymous

And c?

12. anonymous

by-bye polpok

13. anonymous

c is 4

14. anonymous

0=-16^2+80t+4 simplified its 0=-4t^2+20t+1

15. anonymous

Yep.

16. anonymous

but i need t

17. anonymous

Ok so using the formula for the quadratic, what is a, b, and c.

18. anonymous

$0 =at^2 + bt + c \implies t = \frac{-(b) \pm \sqrt{(b)^2 - 4(a)(c)}}{2(a)}$

19. anonymous

a=-4 b=20 c=1

20. anonymous

yea but idk y i keep getting really big numbers wat do u get

21. anonymous

t has 2 be 5.1s or 1s or 2s,3s

22. anonymous

I got -0.049509756796392246, and 5.049509756796392

23. anonymous

Are you forgetting to divide by -8?

24. anonymous

tell me exactly how 2 put it in my calculator cuz everytime i do i get large #'s and yes i put divide y -8

25. anonymous

26. anonymous

so dont put -20 first

27. anonymous

20*20 - 4*(-4)*(1)

28. anonymous

What do you get for that part

29. anonymous

416

30. anonymous

Now take the square root of that

31. anonymous

20.3

32. anonymous

Ok, now you have 2 solutions: $x = \frac{-20 \pm 20.3}{2(-4)}$

33. anonymous

Oh, and you didn't round up.. I think it's 20.396 or 20.4

34. anonymous

im still getting 50.75

35. anonymous

-20 + 20.4 = ? -20 - 20.4 = ?

36. anonymous

2(-4) = ?

37. anonymous

.4

38. anonymous

-8

39. anonymous

ok so u dont multiply the -20 to the 20.4/-8

40. anonymous

No, you add and subtract them

41. anonymous

(-20 plus or minus 20.4) / -8

42. anonymous

i got -0.5

43. anonymous

There's one more possibility.

44. anonymous

nope i got -17.45

45. anonymous

and -22.55

46. anonymous

(-20 - 20.4)/-8

47. anonymous

-0.05 is one option.

48. anonymous

But the other one is positive

49. anonymous

ok i got u now wow thanks but its weird how i cant put the whole thing in the calculator

50. anonymous

Because the order matters

51. anonymous

Plus plugging the whole thing in at once makes it harder to find where you made your mistake. Just put in each piece at a time.

52. anonymous

ok can u help me wit another one

53. anonymous

probably

54. anonymous

using the same formula a flare is launched from the deck of a life boat 4ft above the water surface the initial upward velocity is 80ft/s after how many seconds will the flare be 100 ft above the water surface

55. anonymous

So how would you set this one up?

56. anonymous

h(t)=-16t^2+vt+c where h is the approximate height in ft t is the time in motion in seconds v is the initial upward velocity in ft per second and c is the initial height in ft

57. anonymous

i got 100t=-16t^2+80t+4

58. anonymous

it's just 100, h is the height.

59. anonymous

so $100 = -16t^2 + 80t + 4$

60. anonymous

ok wat next

61. anonymous

move the 100 to the other side so it equals 0 and then use the quadratic formula again.

62. anonymous

by subtracting 100 from both sides that is

63. anonymous

0=-4t^2+20+24

64. anonymous

4-100 is -196, not positive

65. anonymous

i got 3

66. anonymous

err -96 rather

67. anonymous

well den dis aint working now idk u do it

68. anonymous

You did it right, I don't know how, but 3 is one answer. There is another answer

69. anonymous

If you used -4t^2+20+24 You should have gotten (-1 and 6) though

70. anonymous

But the correct formula -4t^2+20t-24 = 0 gives one of the solutions being 3 and the other being 2

71. anonymous

the answer can only be 1s or 2s,3s

72. anonymous

show me ur formula

73. anonymous

$0=-4t^2 + 20t - 24 \implies t = \frac{-20 \pm \sqrt{20^2 - 4(-4)(-24)}}{2(-4)}$

74. anonymous

ok i got dat but now getting big numbers again

75. anonymous

Do the part under the square root first

76. anonymous

$$20^2 - 4(-4)(-24)$$

77. anonymous

16

78. anonymous

and the square root of 16 is?

79. anonymous

4

80. anonymous

Ok, and what's the denominator? 2(-4)

81. anonymous

-8

82. anonymous

polpak, i have some confusion. check your equation once again

83. anonymous

So you have two solutions: (-20 + 4)/-8 and (-20 - 4)/-8

84. anonymous

for the first one you have -16/-8 and the second is -24/-8

85. anonymous

omg thanks

86. anonymous

Hrm India?

87. anonymous

oh whoopse

88. anonymous

No.. wait, that's right

89. anonymous

ok linda dis is my question create ur own i need help 2 get off mines

90. anonymous

no, its alright now. i don't how but the latex code was not properly loaded and the $-4t ^{2}+20t-24$ was looking like $-4t ^{2}-20t-24$

91. anonymous

ah. ok

92. anonymous

a basketball player shoots the ball with an initial velocity of 20ft/s the ball is 6ft above the floor wen it leaves her hands how long will it take for the ball to reach the rim of the basket 10 ft above th floor on its way down

93. anonymous

can u check the answer i get

94. anonymous

i cant do $\sqrt{-21}$

95. anonymous

so how do i do dis problem

96. anonymous

it is a imaginary number!!!! it will come $i \sqrt{21}$

97. anonymous

does dat equal -1

98. anonymous

or 1

99. anonymous

$-1 = i ^{2}$

100. anonymous

i=1 right

101. anonymous

no, it ain't right. i is nothing but an imaginary number. it can not be solved. it has to be written i only.

102. anonymous

well it has 2 be possible cuz there is no i answer on dis paper

103. anonymous

one thing, time can't be negative.

104. anonymous

105. anonymous

i have 0=-4t^+5-1

106. anonymous

$-4t ^{5}-1$

107. anonymous

do you want to write this equation???

108. anonymous

109. anonymous

i know the answer has 2 be 1 or 1.65 but idk y it aint working

110. anonymous

still something wrong in the equation. there is no "t"

111. anonymous

0=-4t^2+5t-1

112. anonymous

yes, the equation is giving t = 1 and o.5

113. anonymous

how uget dat did u use the quadratic formula

114. anonymous

hmm, yes

115. anonymous

how cuz i got -41/-8 + or - 5

116. anonymous

you must be wrong cuz i used calculator to solve.

117. anonymous

i did 2 show me wat u put in

118. anonymous

$\frac{-b \pm\sqrt{b^2-4ac}}{2a}$

119. anonymous

i know dat wat numbers did u end up wit

120. anonymous

1 and 0.5

121. anonymous

122. anonymous

no i mean like wat was b^2-4ac

123. anonymous

0=-4t^2+5t-1 put a = -4 b = 5 and c = -1

124. anonymous

i did i keep getting -41 on top

125. anonymous

$b ^{2}-4ac = 5^{2}-4(-4)(-1)$

126. anonymous

you will get 9

127. anonymous

i was putting -5^2

128. anonymous

-5^2 will also be 25. the sign doesn't matter when there is a square

129. anonymous

thanks

130. anonymous

my pleasure!!!

131. anonymous

0=-16t^2+27+1 need some help