If a and b are two odd positive integers, by which of the following integers is (a4 - b4) always divisible?
3
6
8
12
If X * Y = X2 + Y2 - XY, then 11 * 13 is
117
147
290
433
B.
147
X * Y = X2 + Y2 - XY
∴ 11 * 13 = 112 + 132 - 11 x 13
= 121 + 169 - 143 = 290 - 143 = 147
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