@longrob 

I am modelling demand (X[1]) and cost (X[2]) as my stochastic parameters.

Both of them have normal distribution.To prevent getting negative data we assume always (mu>3*sigma).

Also there is a positive correlation between them(rho).

So there is a bivariate normal distribution as follows:

N(mu1,mu2,sigma 1,sigma2,rho).

Thanks to all;

I should mention that both random variables have positive and non-zero value.

It means that both mu1 and mu2 are greater than zero.

Also to prevent negative value, we assume that (mu>3*sigma) in both normal random variables. 

With these assumptions what is your idea about my questions?

thank you Markiyan for your prompt answer.

My other question is how can we check the concavity of a function in Maple both mathemativcally and graphically?

thank you Markiyan for your prompt answer.

My other question is how can we check the concavity of a function in Maple both mathemativcally and graphically?

First of all Many thanks for your attention.

Actually I used one source by "Igor Hlivka" to write this code in maple.

Here I put the maple file:

bivariate_model_.mws

Download bivariate_model_.mws

At the end of this code you see one function("functionmodel") which is my problem and my question is this:

I want to take first partial derivative of "functionmodel" to "Q" respectively. Then I should put its result equal to "Zero" to find the optimum "Q" as well.

Thank you again for your support.