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CS Grade 6-8 Answer Key

CS: Boolean Logic and Logic Gates

Using true and false values to model digital decisions

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CS: Boolean Logic and Logic Gates

Using true and false values to model digital decisions

CS - Grade 6-8

Instructions: Read each problem carefully. Use true or false values as directed. Show your work in the space provided.
  1. 1

    In Boolean logic, a value can be either true or false. A variable A is true. What is NOT A?

    NOT means the opposite of the given value.

    NOT A is false because the NOT operator changes a Boolean value to its opposite.
  2. 2

    Evaluate the expression: true AND false.

    The expression true AND false is false because an AND expression is true only when both inputs are true.
  3. 3

    Evaluate the expression: true OR false.

    OR needs one or more true inputs.

    The expression true OR false is true because an OR expression is true when at least one input is true.
  4. 4

    Complete this truth table for A AND B: A = true, B = true. What is the output?

    The output is true because both A and B are true, and an AND gate outputs true only when all inputs are true.
  5. 5

    Complete this truth table for A AND B: A = true, B = false. What is the output?

    For AND, every input must be true.

    The output is false because one input is false, and an AND gate needs both inputs to be true.
  6. 6

    Complete this truth table for A OR B: A = false, B = false. What is the output?

    The output is false because both inputs are false, and an OR gate needs at least one true input.
  7. 7

    Complete this truth table for A OR B: A = false, B = true. What is the output?

    For OR, one true input is enough.

    The output is true because B is true, and an OR gate outputs true if at least one input is true.
  8. 8

    A light turns on only when switch A is on and switch B is on. Which logic gate models this rule: AND, OR, or NOT?

    The AND gate models this rule because the light turns on only when both switch A and switch B are on.
  9. 9

    A school website allows login if a user has a password OR a one-time code. Which logic gate models this rule: AND, OR, or NOT?

    The word or in the rule is a clue.

    The OR gate models this rule because the user can log in when at least one of the two conditions is true.
  10. 10

    Evaluate the expression: NOT false AND true. Use normal order where NOT happens before AND.

    The expression is true. NOT false becomes true, and then true AND true equals true.
  11. 11

    Evaluate the expression: NOT (true OR false).

    Solve inside the parentheses first.

    The expression is false. First, true OR false equals true, and then NOT true equals false.
  12. 12

    A game door opens when a player has a key AND has completed the puzzle. If hasKey = true and puzzleComplete = false, does the door open? Explain using Boolean logic.

    The door does not open because true AND false equals false. Both conditions must be true for the door to open.
LivePhysics™.com CS - Grade 6-8 - Answer Key

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