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The symbol and truth table of AND gate are shown below.

                                    A                 A . B                  A                 A . B . C
                                                                             B
                                    B                                        C
                                      Symbol of AND gate

                                    A          B          A.B             A        B       C        A.B.C
                                    0          0           0              0        0       0         0
                                    0          1           0              0        0       1         0
                                    1          0           0              0        1       0         0
                                    1          1           1              0        1       1         0
                              Truth table of two variables AND gate       1        0       0         0
                                                                          1        0       1         0
                                                                          1        1       0         0
                                                                          1        1       1         1
                                                                      Truth table of three variables AND gate
                 2.7.3 OR Gate
                 A logical gate that results in inclusive disjunction is called an OR gate. The OR gate is a binary gate. It produces high
                 output (1) when any one input is high (1). Only when all the inputs are low, 0 is produced as an output. The OR
                 operation is represented by a plus (+). The symbol and truth table of OR gate are shown below.
                                       A                 A+B                A                A+B+C
                                                                            B
                                       B                                    C
                                         Symbol of OR gate                          Symbol

                                    A          B          A+B             A        B       C       A+B+C
                                    0          0           0              0        0       0         0
                                    0          1           1              0        0       1         1
                                    1          0           1              0        1       0         1
                                    1          1           1              0        1       1         1
                              Truth table of two variables OR gate        1        0       0         1
                                                                          1        0       1         1
                                                                          1        1       0         1
                                                                          1        1       1         1
                                                                      Truth table of three variables OR gate

                 Two more logic gates are obtained when AND and OR gates are inverted. They are named as NAND and NOR gates
                 respectively. They are also called universal gates because any digital circuit can be implemented using NAND and NOR
                 gates only.

                 2.7.4 NAND Gate
                 The NAND gate is the complement of AND gate. It produces low output (0) only when all the inputs are high (1). In all
                 other cases, high output (1) is produced. The symbol and truth table of NAND gate are shown below.
                                                                              A
                                       A                   A . B              B               A.B.C
                                        B                                     C

                                         Symbol of NAND gate                        Symbol


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