In the given logical circuit, all logic gates are NAND gate.
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Here, the inputs A and B are inverted using NOT gate (made using NAND gate), then Aˉ and Bˉ are applied as inputs in other NAND gate.
So, (A.A)=A and B.B=B
Output at Y is (A.B)=A+B.
The above boolean expression is for OR gate.
Truth table of OR gate is
| A | B | Y |
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 1 |
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