venting ?

I have a 3 1/2" vent and I would like to drain a 1 1/2" sink drain into it. As long as the 1 1/2" is level how far away can it be from the 3 1/2" with out it's own vent? I'm looking at about 3 1/2', is that to far? Should I vent the sink also? Do I just runt the sink vent back into the 3 1/2" vent, make it look like an "O"? Thanks for the help! dino
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