If a and b are two odd positive integers, by which of the following integers is (a4 - b4) always divisible?
3
6
8
12
C.
8
a4 - b4 = (a - b) (a + b) (a2 + b2), where a and b are odd positive integers.
If two positive integers are odd, then their sum, difference and sum of their squares always even.
∴ (a - b) (a + b) and (a2 + b2) are divisible by 2.
Hence (a - b) (a + b) x (a2 + b2) = a4 - b4 is always divisible by 23 = 8.
If a and b are odd numbers, then which of the following is even?
a + b + ab
a + b - 1
a + b + 1
a + b + 2ab