10 was in the back, so solutions looked pretty uniform!
#9 was interesting:
First of all, when giving an interpretation (in which the
wff is false), you need to specify
The domain
The meaning of Q(x,y)
The meaning of any constants (there are none in this case)
Q(x,y) is a wff, and should not be written as, for example,
Q(x,y) = x+y = 0
Eschew the equal sign! I call this "equality abuse."
Q(x,y) might be written as
Q(x,y): x+y = 0
using a colon, perhaps read "Q(x,y) says that x+y=0"
You are not free to restrict x and y to
different parts of the domain, or different domains
entirely. When we specify a domain, it pertains to all variables.
There is a similar problem on page 49, Example 28. Have a look at that.
