等差数列的和=(a1+an)*n/2
2+4+6+8+...+100=(2+100)*50/2=2550
1+3+5+7+...+99=(1+99)*50/2=2500
1901+1902+1903+1904+...+1999=(1901+1999)*99/2=193050
2+4+6+8+...+100
=(2+100)*50/2=2550
1+3+5+...+99
=(1+99)*50/2=2500
1901+1902+1903+...+1999
=(1901+1999)*49+1950=193050