CARRY adder

Question

number of AND gates for a carry look ahead adder of n bits = 1 + 2 + 3 +................n

i read it somewhere but not sure where

is that statement correct

if yes can i have a reason for it being correct

Comments

Yes it is true

n(n+1)/2 AND gates and n OR gates

for understanding see the diagram here, http://www.eng.ucy.ac.cy/theocharides/Courses/ECE210/Carrylookahead_supp4.pdf

You'll get exact idea

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is it considering AND gates to be only 2 input ?