what is the common difference 1/2 1/3 1/6

Answer 1

Assuming that 1/2, 1/3, and 1/6 are supposed to form an AP
1/3 - 1/2 = 1/6
1/6 - 1/3 = -1/6

there is no common difference.

Answer 2

1/3 - 1/2 = -1/6
so the common difference is -1/6

easy to see if you use a common denominator ...
3/6, 2/6, 1/6, ...

Answer 3

oobleck, thanks for keeping an eye on my responses.
Making quite a few of those silly mistakes lately.