\textsc {Sat} contains \textsc {CNF-Sat} and \textsc {3Sat} as special cases; hence, by restriction, \textsc {Sat} is NP-hard. To see that the general form of \textsc {Sat} is in NP, note that we can guess an assignment of 0's and 1's to the variables in the formula and test in polynomial time if this assignment satisfies the formula.