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How   this is true ?

$_{r}^{\frac{n(n-1)}{2}}\textrm{C} = 2^{\frac{n(n-1))}{2}}$
asked | 44 views
Not getting the question.

some information is missing
i think it should be

r=n(n-1)/2
∑               n(n-1)/2 Cr =2 n(n-1)/2
r=0

yes. can u pls tell how it is true?
it is like all possible subset of n(n-1)/2