How to use Membership API methods in custom classes?

I want to use the Membership API methods (eg:Membership.ValidateUser() etc.) in my class library project custom classes. But, I am not able to use the System.Web.Security namespace. Can't the Membership API methods be used in custom classes? 
I tried to add a reference to the System.Web.Security.dll in my class library project, but wan;t able to locate the DLL.
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