# Compound Propositions and Useful Rules

Type classification: this is a lesson resource. |

Welcome! This is a lesson in the Introductory Discrete Mathematics for Computer Science course here at Wikiversity.

Previous lesson: Biconditional Operator

## Compound Propositions[edit | edit source]

- A
**compound proposition**is a proposition that involves the assembly of multiple statements. This concept was also discussed a bit in the previous lesson.

## Writing Truth Tables For Compound Propositions[edit | edit source]

To write the truth table for a compound proposition, it's best to calculate the statement's truth value after each individual operator. For example, in the statement , it's best to solve for , then for , and finally for the statement as a whole:

p | q | |||
---|---|---|---|---|

T | T | F | T | T |

T | F | T | T | F |

F | T | F | F | T |

F | F | T | T | F |

## Showing Logical Equivalence[edit | edit source]

Logical equivalence means that the truth tables for two statements are the same. This was also discussed a bit in the previous lesson. In order to prove logical equivalence, simply draw the truth tables for all the statements in question and show that they are the same.

For example, you can show that is logically equivalent to .

p | q | |||
---|---|---|---|---|

1 | 1 | 0 | 1 | 1 |

1 | 0 | 0 | 0 | 0 |

0 | 1 | 1 | 1 | 1 |

0 | 0 | 1 | 1 | 1 |

## The Contrapositive, Inverse and Converse[edit | edit source]

The **contrapositive** of conditional statement is . **A conditional is logically equivalent to its contrapositive.** In other words, if q did not occur, then we can assume p also did not occur.

The **inverse** is .

The **converse** is .

The **converse** and **inverse** are logically equivalent. They are one another's contrapositives...

## Tautology[edit | edit source]

A **tautology** is a statement that is always true. Another wording is that a tautology is a statement that is logically equivalent to the constant truth.

is an example of a tautology.

## Congratulations[edit | edit source]

Congratulations! You have finished the first lesson of the course.