How to calculate sd when dividing two means with their sd's?

I have group A with N=10 and group B with N=12 and have to divide group A by B (I calculate results from Real Time PCR). I am not able to divide n's from group A by n's from group B because groups contain different numbers of reads (N number). So I want to divide mean of group A by mean of group B. That's easy, by what with sd (standard deviation)? I have calculated sd for group A and group B, but how calculate final sd? Please, explain to me very simple, because I not very good at this. Maybe You can give me some link to software which can calculate this?

Please, help
Thanks, Ola

redlugi said:

I have group A with N=10 and group B with N=12 and have to divide group A by B (I calculate results from Real Time PCR). I am not able to divide n's from group A by n's from group B because groups contain different numbers of reads (N number). So I want to divide mean of group A by mean of group B. That's easy, by what with sd (standard deviation)? I have calculated sd for group A and group B, but how calculate final sd? Please, explain to me very simple, because I not very good at this. Maybe You can give me some link to software which can calculate this?

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