Complete Question
The complete question is shown on the first uploaded image
Answer:
a) The required additional minterms for f so that f has eight primary implicants with two literals and no other prime implicant are
and 
b) The essential prime implicant are
and 
c) The minimum sum-of-product expression for f are
Explanation:
The explanation is shown on the second third and fourth image
Answer:
Opened Push-button Switch (i.e. a PTM Switch)
Explanation:
Tha's just another symbol for a switch, but this one specifies that the switch is a push-button type of switch.
Since it's not touching and completing the line, the state of the switch is initially open.