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Kisachek [45]
3 years ago
12

According to a study conducted by the Toronto-based social media analytics firm Sysomos, of all tweets get no reaction. That is,

these are tweets that are not replied to or retweeted (Sysomos website, January ). Suppose we randomly select tweets.a. What is the expected number of these tweets with no reaction (to the nearest whole number)
a) What is the expected number of these tweets with no reaction?
b) What are the variance and standard deviation for the number of these tweets with no reaction?
Mathematics
1 answer:
Svetllana [295]3 years ago
5 0

Answer:

a) E(X) = 71

b) V(X) = 20.59

Sigma = 4.538

Step-by-step explanation:

<em>The question is incomplete:</em>

<em>According to a 2010 study conducted by the Toronto-based social media analytics firm Sysomos, 71% of all tweets get no reaction. That is, these are tweets that are not replied to or retweeted (Sysomos website, January 5, 2015). </em>

<em> Suppose we randomly select 100 tweets. </em>

<em>a) What is the expected number of these tweets with no reaction? </em>

<em>b) What are the variance and standard deviation for the number of these tweets with no reaction?</em>

This can be modeled with the binomial distribution, with sample size n=100 and p=0.71, as the probability of no reaction for each individual tweet.

The expected number of these tweets with no reaction can be calcualted as the mean of the binomial random variable with these parameters:

E(X)=n\cdot p=100\cdot 0.71=71

The variance for the number of these tweets with no reaction can be calculated as the variance of the binomial distribution:

V(X)=np(1-p)=100\cdot0.71\cdot0.29=20.59

Then, the standard deviation becomes:

\sigma=\sqrt{V(X}=\sqrt{20.59}=4.538

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given

f'(x) = 12cos x - 4sin x, then

f'(\frac{4\pi }{3}) = 12cos(\frac{4\pi }{3}) - 4sin(\frac{4\pi }{3})

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A Norman window has the shape of a rectangle surmounted by a semicircle. Suppose the outer perimeter of such a window must
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The base length that will maximize the area for such a window is 168.03 cm. The exact largest value of x when this occurs is 233.39 cm

Suppose we make an assumption that:

  • (x) should be the width of the rectangle base;
  • (h) should be the height of the rectangle

Also, provided that the diameter of the semi-circle appears to be the base of the rectangle, then;

  • the radius  \mathbf{r = \dfrac{x}{2}}  

and, the perimeter of the window can now be expressed as:

\mathbf{x + 2h + \pi r = x + 2h + \dfrac{\pi x }{2}}

\mathbf{= \Big ( 1 + \dfrac{\pi}{2}\Big) x + 2h}

Given that the perimeter = 600 cm

∴

\mathbf{ \Big ( 1 + \dfrac{\pi}{2}\Big) x + 2h= 600}

\mathbf{  h = 300 - \Big( \dfrac{1}{2} + \dfrac{\pi}{4}\Big) x}

Since h > 0, then:

\mathbf{  h = 300 - \Big( \dfrac{1}{2} + \dfrac{\pi}{4}\Big) x>0}

By rearrangement and using the inverse rule:

\mathbf{  x<  \dfrac{ 300}{\Big( \dfrac{1}{2} + \dfrac{\pi}{4}\Big) } }

\mathbf{  x=  \dfrac{ 1200}{\Big( 2 +\pi \Big) } }

\mathbf{  x=  233.39 \ cm }

Thus, the largest length x = 233.39 cm

However, the area of the window is given as:

\mathbf{A(x) = xh + \dfrac{1}{2} \pi r^2}

\mathbf{A = x \Big [  300 - \Big ( \dfrac{1}{2}+\dfrac{1}{4} \Big) x \Big ]  +\dfrac{1}{2}\pi \Big(\dfrac{x}{2} \Big )^2}

\mathbf{A (x) = 300x - \Big( \dfrac{1}{2} + \dfrac{\pi}{8}\Big) x^2 \ cm^2}

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\mathbf{\dfrac{d}{dx} 300x - \dfrac{d}{dx} \Big( \dfrac{1}{2} + \dfrac{\pi}{8}\Big) x^2 \ =0}

\mathbf{ 300 - 2x \Big( \dfrac{1}{2} + \dfrac{\pi}{8}\Big)  \ =0}

Making x the subject of the formula, we have:

\mathbf{x = \dfrac{1200}{4 +\pi}}

x = 168.03 cm

Taking the second derivative:

\mathbf{\dfrac{d}{dx} \Big [300 -2x \Big( \dfrac{1}{2} + \dfrac{\pi}{8}\Big) \Big]}

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Therefore, we can conclude that the maximum area that exists for such a window is 168.03 cm

Learn more about derivative here:

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