Question

show that positive logic nand gate is negative logic nor gate

Answer 1

Not (A and B) = Not (A) or Not (B) by use of De Morgan theorem

Answer 2

let A & B be two inputs to a NAND gate and Y be output
A B Y
L L H
L H H
H L H
L L H

In a negative logic circuit L(low voltage level) represents logic '1' and H(high voltage level) represents logic '0' and conversely for positive logic circuits.
So for positive logic you put L=0 and H=1 you will get the table for positive logic NAND gate .If now you put H=0 and L=1,you will get a table for NOR gate and obviously it is in negative logic.