How to use LT3757 for both positive and negative voltages.

It is stated in the LT3757's data sheet it may "either [generate] positive or negative output voltage with a single FBX pin". I am hoping to generate -20V to +20V (IL=0.01A) by adjusting the feedback resistors R2 and R1. But i do not understand how to go below 0V. Can someone explain? Thanks!

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