"vary" means to take on a different values , to change in value -- pretty much the everyday meaning.
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It means: 3 times n
x-n = 1 / xn
I assume that with n(...) you mean the size of the set. It sure can; but it need not be.
n will typically mean number. It can stand for any given number, and what it stands for will depend on the problem.
The equation N-6=2 is a math problem. The answer to this math problem would be N=4.