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in Probability
Sep 28, 2014

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33 votes

Let $S$ be a sample space and two mutually exclusive events $A$ and $B$ be such that $A \cup B = S$. If $P(.)$ denotes the probability of the event, the maximum value of $P(A)P(B)$ is_____.

7

A U B = S → P(A U B) = P(S) = 1

P(A U B) = P(A) + P(B) – P(A ${\cap }^{}$ B) [ P(A ${\cap }^{}$ B) = Phi,as they are mutually exclusive events]

So, P(AUB) = P(A) + P(B) = 1

P(A)xP(B) = P(A)x(1-P(A)) [ as P(A) + P(B) = 1]

Put, P(A) = Z , P(A)xP(B) = P(A)x(1-P(A)) = Zx(1-Z)

Maximum value of P(A)xP(B) = Maximum value of Zx(1-Z) = f(Z)

The maximum value of f(Z) will occur when f**’**(Z) =0 [f**’**(Z) = Derivation of f(Z) with respect to Z]

Now f**’**(Z) = 1-2xZ = 0 →** Z = 1/2**

So, P(A)xP(B) = Zx(1-Z) will be maximum when Z = ½

**So, Maximum value of P(A)xP(B) = Zx(1-Z) = (1/2)x(1-1/2) = ¼ = 0.25 **

3

40 votes

Best answer

Visual Representation for Clarity:-

Since $A \cup B = S$. So, we can partition $S$ into two parts $A$ and $B$.

We know that max value of product of $P(A)$ and $P(B)$ can happen only if they are equal and since $P(A)+P(B) = S$, so $P(A) = P(B) = 0.5$.

[If they were like $70\%$ and $30\%$, then $P(A) * P(B) = 0.7*0.3 = 0.21$ which is $< (0.5)*(0.5) = (0.25)$]

2

( Just for clarity purpose )

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Here, P(A) + P(B) = 1

Let S = P(A).P(B)

Therefore, S = P(A)(1 – P(A))

S = P(A) – P(A)^2

Differentiating, both sides w.r.t. P(A) –

dS/dx = 1 – 2P(A)

for maximize the value of s, dS/dx = 0

1 – 2P(A) = 0

P(A) = ½

therefore, S = ½ * ½ = ¼

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Here, P(A) + P(B) = 1

Let S = P(A).P(B)

Therefore, S = P(A)(1 – P(A))

S = P(A) – P(A)^2

Differentiating, both sides w.r.t. P(A) –

dS/dx = 1 – 2P(A)

for maximize the value of s, dS/dx = 0

1 – 2P(A) = 0

P(A) = ½

therefore, S = ½ * ½ = ¼

1

7 votes

Sample Space(S) - A set of all possible outcomes/events of a random experiment. Mutually Exclusive Events - Those events which can't occur simultaneously. P(A)+P(B)+P(A∩B)=1 Since the events are mutually exclusive, P(A∩B)=0.

Therefore, P(A)+P(B)=1

Now, we now that AM >= GM So, (P(A)+P(B))/2 >= sqrt(P(A)*P(B)) P(A)*P(B) <= 1/4

Hence max(P(A)*P(B)) = 1/4.

We can think of this problem as flipping a coin, it has two mutually exclusive events ( head and tail , as both can't occur simultaneously). And sample space S = { head, tail } Now, lets say event A and B are getting a "head" and "tail" respectively. Hence, S = A U B. Therefore, P(A) = 1/2 and P(B) = 1/2. And, P(A).P(B) = 1 /4 = 0.25. Hence option B is the correct choice.

Therefore, P(A)+P(B)=1

Now, we now that AM >= GM So, (P(A)+P(B))/2 >= sqrt(P(A)*P(B)) P(A)*P(B) <= 1/4

Hence max(P(A)*P(B)) = 1/4.

We can think of this problem as flipping a coin, it has two mutually exclusive events ( head and tail , as both can't occur simultaneously). And sample space S = { head, tail } Now, lets say event A and B are getting a "head" and "tail" respectively. Hence, S = A U B. Therefore, P(A) = 1/2 and P(B) = 1/2. And, P(A).P(B) = 1 /4 = 0.25. Hence option B is the correct choice.

5 votes

When P(A)P(B) is maximum A,B together should complete whole universal space( P(A∪B)=1, also in the question it is mentioned that A∪B=S ,so P(A U B)=1) and P(A),P(B) not equal to 0

It is given that A,B are mutually exclusive events(P(A∪B)= P(A) + P(B) ). So, if we let P(A)=x then,

P(A∪B)= P(A) + P(B) or, 1= x +P(B) or, P(B)=1-x. Hence, P(A)P(B)= x.(1-x)

derivative of x(1-x)= 1-2x, So, when P(A)P(B) is maximum, 1-2x=0 or, x=1/2, Hence x(1-x)= 1/2 * 1/2 = 1/4=0.25

It is given that A,B are mutually exclusive events(P(A∪B)= P(A) + P(B) ). So, if we let P(A)=x then,

P(A∪B)= P(A) + P(B) or, 1= x +P(B) or, P(B)=1-x. Hence, P(A)P(B)= x.(1-x)

derivative of x(1-x)= 1-2x, So, when P(A)P(B) is maximum, 1-2x=0 or, x=1/2, Hence x(1-x)= 1/2 * 1/2 = 1/4=0.25