Q.Write the circuit symbol and the truth table of a three-input NOR gate.

NOR = OR + NOT, so its symbol is the OR-gate shape with an inversion bubble at the output.

The NOR gate performs the Boolean operation Y = (A + B)' — it is an OR gate followed by an inverter (NOT gate). Its standard circuit (logic) symbol is therefore built from the OR-gate symbol:

• Two inputs A and B enter from the left into a shape with a curved (concave) back and a curved, pointed front — exactly the standard OR-gate shape.
• A small circle (the 'inversion bubble') is drawn at the pointed output tip.
• The single output Y is taken from beyond this bubble. …

NOR = NOT of OR, so Y = ‾(A + B).

The OR operation is written A + B. A NOR gate inverts this output, so its Boolean expression is:

$Y = \overline{A + B}$

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A NOR gate is drawn as an OR gate symbol with a small inversion bubble (circle) at its output — it performs OR then NOT.

The standard logic symbol: two inputs A, B enter a curved-back, pointed OR-gate shape; a small circle is placed at the tip (output) of the gate to denote inversion. Boolean expression: $Y = \overline{A+B}$.

Truth table:

A=0,B=0 → Y=1 …

NOR = OR + NOT: draw the OR-gate symbol (curved-back, pointed-nose shape) with a bubble at the output; output $Y=\overline{A+B}$.

Concept. A NOR gate gives output 1 only when all inputs are 0; it is the OR gate with its output inverted.

The logic symbol (description).

• Two input lines A and B enter the flat/curved back of an OR-gate shape.
• The body is the OR-gate shape: a concave (curved) input side tapering to a pointed output nose. …

A curved-back OR-gate-shaped symbol with a small inversion bubble on its output is the standard symbol for a NOR gate.

Logic gate symbols are identified by their body shape plus the presence/absence of an output bubble:

• A curved (concave) back with a pointed front = OR-gate body (output HIGH if A OR B is HIGH).
• A flat back with a rounded/D-shaped front = AND-gate body.
• A small circle (bubble) on the output line means the output is inverted/complemented relative to the plain gate. …

NOR = OR gate symbol + a small inversion bubble at the output.

A NOR gate is a NOT-OR gate: it performs the OR operation on its inputs and then inverts (complements) the result, so its output is HIGH only when all inputs are LOW.

Symbol (described): it is drawn exactly like the standard 2-input OR gate — a shield-like body with a concave (curved-in) input side where the two input lines $A$ and $B$ enter, and a pointed, curved output side — but with a small circle ('bubble') placed at the pointed output tip. This bubble is the standard logic-symbol convention for inversion/complementation, distinguishing NOR from a plain OR gate.

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