S1: Discrete Random Variable... How to find the variance?

I don't understand question d)i) Where it asks me to find the variance. I found the Var(X) and the mean of Y. But I don't understand the markscheme where it asks me to find the Var(Y). It's been a long time and I have forgotten the techqiue.

According to the markscheme, you multiply the square of 4 by the var(X). But why do you square the 4???? :s-smilie:

Original post by Future_Dr

I don't understand question d)i) Where it asks me to find the variance. I found the Var(X) and the mean of Y. But I don't understand the markscheme where it asks me to find the Var(Y). It's been a long time and I have forgotten the techqiue.

According to the markscheme, you multiply the square of 4 by the var(X). But why do you square the 4???? :s-smilie:

Don't you have a result in your textbook that says

Var(aX + b) = a^2 Var(X)

, where a and b are constants?

Reply 2

mynameisntbobk

18

Original post by Future_Dr

I don't understand question d)i) Where it asks me to find the variance. I found the Var(X) and the mean of Y. But I don't understand the markscheme where it asks me to find the Var(Y). It's been a long time and I have forgotten the techqiue.

According to the markscheme, you multiply the square of 4 by the var(X). But why do you square the 4???? :s-smilie:

Generally, Var(2x) = 2^2 Var(X)

Similarly, Var(Y) = Var(4X+2) = 4^2 Var(X)

(edited 11 years ago)

Reply 3

username854791

OP

14

Thanks guys.

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