Answer from cs61c-eq (Nicholas Hwang 16732453) for Question 3
They represent "not-cares," or values that have no relevance in certain situations. 
The sum of products is : !a + a * !b
No, this table is complete. If the left bit is zero, output is always one. If the left bit is not zero, it only depends on the last bit; the middle bit is of no consequence.