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## Homework Statement

the question asks you to calculate the standard derivation for the mean distance of an electron from the nucleus.

you are given the mean distance (<r>), and the probability density

## Homework Equations

delta r = sqrt (<r^2> - <r>^2)

<r> = 3.a/2

## The Attempt at a Solution

We had to calculate <r^2> by integrating a given value.

after integrating by parts 4 times i ended up with -3a^2

putting this into the standard derivation lead to the square root of minus 21.a^2/4

this obviously gives a complex value.

i was wondering if this can be correct, or if the negative sign has no meaning in the calculation.

i have read from some physics books but i cant find an answer