根号1/7=(根号7)/7
根号28=根号(4*7)=2又根号7
根号700=(根号7*100)=10又根号7
根号1/7+根号28-根号700
=(根号7)/7+2*根号7-10*根号7
=(1/7+2-10)又根号7
=7又6/7*根号7