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You play a game where you first choose a positive integernand thenflip a fair coinntimes. You win a prize if you get exactly 2 heads. How should youchoosento maximize your chance of winning

Sagot :

Solution :

The probability of winning when you choose n is =  [tex]$^nC_2\left(\frac{1}{2}\right)^n$[/tex]

[tex]$n\left(\frac{n-1}{2}\right)\times \left(\frac{1}{2}\right)^n = n(n-1)\left(\frac{1}{2}\right)^{n+1}$[/tex]

Apply log on both the sides,

[tex]$f(n) = \log\left((n)(n-1)\left(\frac{1}{2}\right)^{n-1}\right) = \log n +\log (n-1)+(n+1) \ \log\left(\frac{1}{2}\right)$[/tex]

Differentiation, f(x) is [tex]$f'=\frac{1}{x}+\frac{1}{(x-1)}+\log\left(\frac{1}{2}\right)$[/tex]

Let us find x for which f' is positive and x for which f' is negative.

[tex]$\frac{1}{x}+\frac{1}{(x-1)} > 0.693$[/tex]       , since [tex]$\log(1/2) = 0.693147$[/tex]

For x ≤ 3, f' > 0 for [tex]$\frac{1}{x}+\frac{1}{x-1}+\log\left(\frac{1}{2}\right)>0$[/tex]

[tex]$\frac{1}{x}+\frac{1}{x-1}-0.6931470$[/tex]

That means f(x) is increasing function for n ≤ 3

[tex]$\frac{1}{x}+\frac{1}{x-1}< 0.693147 $[/tex] for x > 4

f' < 0 for n ≥ 4, that means f(n) is decreasing function for n ≥ 4.

Probability of winning when you chose n = 3 is [tex]$3(3-1)\left(\frac{1}{2}\right)^{3+1}=0.375$[/tex]

Probability of winning when you chose n = 4 is [tex]$4(4-1)\left(\frac{1}{2}\right)^{4+1}=0.375$[/tex]

Therefore, we should chose either 3 or 4 to maximize chances of winning.

The probability of winning with an optimal choice is n = 0.375