Question Video: Using Properties of Variance of Random Variable to Solve Problems

Given that Var(𝑋) = 28, find Var(4𝑋 + 9).


Video Transcript

Given that the variance of 𝑋 equals 28, find the variance of four 𝑋 plus nine.

We need to remember that this kind of variance has certain properties. One of those properties looks like this. The variance of 𝐴𝑋 plus 𝐵 is equal to 𝐴 squared times the variance of 𝑋. We can plug in 28 for the variance of 𝑋. And we can substitute the variance of four 𝑋 plus nine in place of 𝐴𝑋 plus 𝐵. When we do this, we see that the 𝐴 value is equal to four. So we plug in four for the 𝐴 value. Four squared times 28 equals 16 times 28, which equals 448.

The variance of four 𝑋 plus nine is equal to 448. We know this is true based on the properties of variances.

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