Step 1: Define Expectation of a Random Variable.
- a) The Expectation (or Expected Value) of a random variable is the weighted average of all possible values that the random variable can take. The weights are the probabilities of each value occurring. It represents the long-run average value of the random variable if the experiment is repeated many times. For a discrete random variable X, it is given by E[X]=∑xP(X=x).
Step 2: State the Properties of Expectation of a Random Variable.
- b) The key properties of the expectation of a random variable are:
- Expectation of a Constant: If c is a constant, then E[c]=c.
- Linearity (Constant Multiple): If c is a constant and X is a random variable, then E[cX]=cE[X].
- Linearity (Sum/Difference): If X and Y are random variables, then E[X±Y]=E[X]±E[Y].
- Expectation of a Sum of Independent Random Variables: If X1,X2,…,Xn are random variables, then E[∑i=1nXi]=∑i=1nE[Xi]. This property holds even if the random variables are not independent.
- Expectation of a Product of Independent Random Variables: If X and Y are independent random variables, then E[XY]=E[X]E[Y].
Step 3: Construct the Probability Distribution Table and find the Expected Value of X.
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c) Let S be the sample space for tossing a fair coin twice. The possible outcomes are:
S={HH,HT,TH,TT}
Each outcome has a probability of 41 since the coin is fair.
The random variable X is defined such that X(e)=0 if e is T (tail) and X(e)=1 if e is H (head). This means X counts the number of heads in the two tosses.
Let's map the outcomes to the values of X:
- HH⟹X=1+1=2
- HT⟹X=1+0=1
- TH⟹X=0+1=1
- TT⟹X=0+0=0
The possible values for X are {0,1,2}.
Now, we find the probability for each value of X:
- P(X=0): Occurs for outcome TT. So, P(X=0)=41.
- P(X=1): Occurs for outcomes HT and TH. So, P(X=1)=41+41=42=21.
- P(X=2): Occurs for outcome HH. So, P(X=2)=41.
The Probability Distribution Table for X is:
xP(X=x)041121241
Now, we calculate the Expected Value of X, E[X]:
E[X]=∑xP(X=x)
E[X]=(0×P(X=0))+(1×P(X=1))+(2×P(X=2))
E[X]=(0×41)+(1×21)+(2×41)
E[X]=0+21+42
E[X]=0+21+21
E[X]=1
The Expected Value of X is 1.
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