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Truth Tables

You use truth tables to predict the output depending on certain inputs. In truth tables you list all possible inputs, and then, based on the gate used, can accurately predict the output. The inputs can be listed as T or F or as 1 (which is TRUE) or 0 (which is FALSE)

Truth table for NOT P

The following image shows a truth table for output ~ P (NOT P). If the input is T, the output is F and if the input is F the output is T.

Truth table for AND

In the following truth table we have TWO inputs, P and Q. In order for the output to be TRUE (T), both inputs need to be T.  This truth table corresponds to the nature of an AND gate. 

Truth table for OR

In this example, note that either one of the inputs needs to be T for the output to be T. 

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Further Reading

Source for images: brilliant. org / wiki/truth-tables