A logic diagram which contains both an AND and an OR operation
is a second-order-logic diagram.
The diagram above is an example of second-order logic, an AND
circuit supplying an OR circuit. The output of each logic symbol
must be determined in order to develop an output Boolean
expression correctly. Start with the inputs to the first symbol and
proceed to the final output. Use the output of each logic symbol as
an input to the next succeeding symbol as follows:
Step 1. The first symbol is recognized as an AND gate with two
inputs, A and B. The output, AB, in turn, is one of the
inputs to the OR gate.
Step 2. The final output from this logic diagram is from the OR
gate. An examination of the output expression reveals
that it is correctly indicating the output from an OR gate
which had three separate inputs, AB, C, and D.