How is radioactive decay used to date sedimentary rocks?

Sedimentary rocks can be dated using radioactive carbon, but since carbon decays  pretty fast, this only works for rocks younger than around 50 thousand years. So to date most older fossils, scientists look for layers of igneous rock or volcanic ash above and below the fossil. Hope that helps.

By measuring the amounts of nitrogen and carbon-14 in the sedimentary rocks.

Explanation:

Distinct radioactive isotopes exhibit distinct half-lives, that is, half of the total time consumed for half of the total isotopes in the substance to decay. Thus, in order to find the age of the sedimentary rock in the radiometric dating, there is a need to find the initial amount of the isotopes (in the given case of nitrogen and carbon-14), and then that decayed one is predicted. The determination of the half-lives of the elements is then utilized to find the age of the fossil or the rock.

the denominator has a very specific role.

"1. it should be, it has been for about 20 years, and it's really not much different in the end than breeding animals to have particular traits. there are always concerns about genetically engineered animals/plants affecting the environment in unexpected negative ways, but i can't think of an example where the fears didn't turn out to be baseless.  

first, as you point out, it would be a great advance in pharmacology, to be able to tailor drugs and their doses to me. the pros involve better medical treatment. the cons involve privacy issues like insurance companies denying coverage to people with genetic risk factors. looking at dna markers wouldn't affect the frequency of genetic diseases, but it would allow detection of them earlier. and ok, actually i suppose that if people used this information to avoid having children with genetic diseases, some might decrease in frequency."