1/3 + 1/6 + 1/10 + ... + 1/x(x+1):2=2001/2003
2/6+2/12+2/20+.....+2/x(x+1)=2001/2003
2×(1/2×3+1/3×4+1/4×5+...+1/x(x+1))=2001/2003
1/2-1/3+1/3-1/4+1/4-1/5+....+1/x-1/x+1=2001/2003÷2
1/2-1/x+1=2001/4006
=>1/x+1=1/2-2001/4006=1/2003
=>x+1=2003
=>x=2003-1
=>x=2002
Chúc bạn hok tốt!