=50(1+2+3......+100)=50X(1+100)X100/2=252500最后一个数是5000
Sn=nA1+[n(n-1)d]/2 =100*50+100*99*50/2 =5000+247500 =252500
（50+5000）*100/2=252500
50＋50×99＝5000（50＋5000）×50＝252500
252500
