# probabilities of normal distribution as a function of unknown standard deviation.

fbeleznay shared this question 1 year ago

I have the following problem.

X follows a normal distribution with mean 10 and unknown standard deviation. I would like to find the standard deviation, for which the probability P(8<X<11) is 0.7.

I tried to graph this probability as a function of the standard deviation, but I got an error message.

I tried:

h(x)=Normal(10,x,11,true)-Normal(10,x,8,true)

Am I doing something wrong? Is it possible to put the unknown in the position of the standard deviation? If not, how can this be done without actually using the formula of the normal curve?

1

update: I sorted this out using the erf function

Nevertheless, I think, an normalcdf(lower bound, upper bound, mean, st.dev) option would be useful, where all four parameters can be set as the variable to solve for.

Similarly with other distributions.

Also, for an integrals: integral(lower, upper, function, variable), where the variable is not necessarily x, so x can appear in the limits and functions.

1

You can probably also use NSolve() in the CAS View

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