1+2+3+4+5+6+7+8+9+10+11+"""""+97+98+99+100 =(1+100)*100/2 =101*100/2 =10100/2 =5050
100+99+98+97+······+3+2+1=100X50+100=101X505050
(1+100)*100/2=5050
(1+100)*100/2=5050
5050