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Yeetww

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 #1
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I suggest you read this similar question

Try figuring it out by yourself.

Dec 18, 2022
 #1
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Compute (53)+(63)+(73).

USE THE HOCKEY STICK THEOREM (See this).

 

 

See that (53)+(63)+(73)=(84)[(43)+(33)]=(84)[(41)+(30)]=70(4+1)=65

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Dec 18, 2022
 #6
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Solution:

(a) The number of ordered triples (a,b,c), where 1a,b,cn, is n3 We can classify these triples into three categories.

Case 1:  a,b,c are all equal.

There are n ordered triples in this case.

Case 2: Two of a,b,c are equal, and the third is different.

There are n ways to choose the two equal values, then n1 ways to choose the third value that is different. There are then 3 ways to arrange the numbers within the ordered triple, so the number of ordered triples in this case is 3n(n1).

Case 3: a,b,c are all different.

There are (n3) ways to choose three different values. There are then 3!=6 ways to arrange them, so the number of ordered triples in this case is 6(n3)

By counting the number of triples (a,b,c) in two different ways, we arrive at the conclusion 

n3=n+3n(n1)+6(n3).

(b) The number of subsets of {1,2,3,...,n+2} containing three different numbers is (n+23)

We can also count the number of subsets as follows. Let the subset be {a,b,c}, where \(a  Then the middle element  b is at least 2 and at most  n+1 so we can let  b=k+1 where  1kn.

Once the middle element  b=k+1 is chosen, the smallest element  a must be between 1 and  k inclusive, so there are  k ways of choosing the smallest element  a. The largest element  c must be between  k+2 and  n+2 inclusive, so there are  (n+2)(k2)+1 ways of choosing the largest element  c. So the number of subsets where the middle element is  k+1 is  k(nk+1) Summing over  1kn we get

(n+23)=(1)(n)+(2)(n1)+(3)(n2)++(k)(nk+1)++(n)(1).
 

Nov 6, 2022
 #9
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Let us call the circle's center O. We first note that if A and B are points on the circle, then triangle AOB is isosceles with AO=BO. Therefore, if AOB is an obtuse triangle, then the obtuse angle must be at O. So AOB is an obtuse triangle if and only if minor arc AB has measure of more than  π2(90).

Now, let the three randomly chosen points be A0,A1, and A2. Let θ be the measure of minor arc A0A1. Since θ is equally likely to be any value from 0 to π, the probability that it is less than π2 is 12.

Now suppose that θ<π2. For the problem's condition to hold, it is necessary and sufficient for point A2 to lie within π2 of both A0 and A0 along the circumference. As the diagram below shows, this is the same as saying that A2 must lie along a particular arc of measure πθ.



The probability of this occurrence is πθ2π=12θ2π, since A2 is equally likely to go anywhere on the circle. Since the average value of θ between 0 and π2 is π4, it follows that the overall probability for θ<π2 is 12π/42π=38.

Since the probability that θ<π2 is 1/2, our final probability is 1238=316.

Oct 22, 2022