i'm trying to do my own Due board , but there's a Circuit i don't understand， there are four
resistances in parallel on the left side , i can't understand what the purpose for those resistances
thanks all .
They have no purpose, they aren't individual resistors which is why they appear in the circuit.
They're spares. I don't know why they have two resistor networks of 4 elements each and each network is the same value as the other and then they use half of each. One would have covered it nicely but I guess the plan is to allow for two different values or two resistors to be a load option without resorting to solder links or jumpers.
I do agree though, that connecting them all to ground seems a little odd. It doesn't appear to be for any appreciable purpose like isolating signals in adjacent resistors to reduce cross talk which is the only thing I can think of by way of a reason. I guess it is just the preference of the designer not to leave them unconnected.
If there was a solder short between two pins I guess it would make it obvious, or at least not ignorable. Curious. 
PE