a,
x+(x+1)+(x+2)+...+(x+100)=100x+6000
x+x+x+...+x +(1+2+3+...+100)= 100x+6000
101x+(1+100)+(2+99)+...+(50+52)=100x+6000
101x+101+101+...101=100x+6000
101x+101.50=100x+6000
101x+5050=100x+6000
100x+x+5050=100x+6000
từ đây ta suy ra :
x+5050=6000
x=6000-5050
x=50