Question

For all positive integers a and b, let be defined by a@b = a^2+ b^2 -2. If c and d are positive integers, which of the following CANNOT be the value of c@d?
a. 0
b. 2
c. 3
d. 6
e. 8

Answer 1

Have you tried combinations of the first few positive integers? I'd suggest beginning with c = 1 and d = 1? And then

Combinations:
C D
1 1
1 2
1 3
2 1
2 2
2 3

until you find the number not able to be a result of c@d.