tu 1-> 9 co so cac so la:
(9-1)/1+1=9(so)
can so cac chu so la:
9 *1=9(chu so) (*)
tu 10->99 co so cac so la:
(99-10)/1+1=90(so)
can so cac chu so la:
90*2=180(chu so) (*)
tu 100->106 co so cac so la:
(106-100)/1+1=7 (so)
can so cac chu so la:
7*3=21(chu so) (*)
tu do ta co:
9+180+21=210(chu so)
tick mik nha bn