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What is the variance of the standard normal distribution?

Answer
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Hint: To find the variance of the standard normal distribution, we will use the formula Var[X]=E[X2]E[X]2 . We can find E[X2] using the formula E[X2]=x2fx(x)dx and substituting for fx(x)=12πe12x2 . Then, we have to integrate by substitution method and apply the properties of Gamma functions. We can find E[X] using the definition of standard normal distribution which will result in a value 0. Then, we have to substitute the values in the variance equation and solve.

Complete step by step solution:
Let us first see what a standard normal distribution is. The standard normal distribution is one of the forms of the normal distribution. It occurs when a normal random variable has a mean equal to zero and a standard deviation equal to one.
Let us find the variance of standard normal random variable X. We know that variance of X is given by
Var[X]=E[X2]E[X]2...(A)
We know that E[X2]=x2fx(x)dx
Where fx(x)=12πe12x2 is the Probability Density Function of a standard normal distribution. Let us substitute this value in the above equation.
E[X2]=12πx2e12x2dx
We can rewrite the above integral as
E[X2]=12πxe12x2xdx...(i)
Let us assume y=x22...(ii) . We have to differentiate equation (ii) with respect to x.
dydx=12×2xdy=xdx...(iii)
Now, from equation (ii), we can find the value of x as
x=2y...(iv)
Let us substitute (ii), (iii) and (iv) in equation (i).
E[X2]=12π2yeydy
Let us take the constants outside.
E[X2]=22πy12eydyE[X2]=1πy12eydy
We know that f(x)dx=20f(x)dx . Therefore, we can write the above integral as
E[X2]=2π0y12eydy...(v)
We can see that 0y12eydy is a Gamma function of the form Γ(a)=0ya1eydy for a>0 . We can write equation (v) as
E[X2]=2π0y(12+1)1eydy
Now, let us convert the integral into Gamma function.
E[X2]=2πΓ(12+1)
We know that Γ(z+1)=zΓ(z) . Therefore, we can write the above equation as
E[X2]=2π12Γ(12)
We know that Γ(12)=π . Therefore, the above equation becomes
E[X2]=2π12×π
Let us cancel the common terms.
E[X2]=1
Let us substitute the above value in equation (A).
Var[X]=1E[X]2
We know that for a standard normal distribution, mean or expectation is 0. Therefore, the above equation becomes
Var[X]=102Var[X]=1
Therefore, the variance of the standard normal distribution is 1.

Note: Students must know the mean of standard normal distribution to find the variance. They must also know to integrate functions and also the values of the integral of basic functions. They must also know about the Gamma distribution or function and few properties related to these.
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