a,  đặt A=-2/6+-2/66+..+-2/46.51

 => A=-2.(1/6+1/66+..+1/46.51)

=>5.A =-2.(5/6+5/66+..+5/46.51)

=>5.A=-2.(5/1.6+5/6.11+...+5/46.51)

=>5.A=-2.(1-1/6+1/6-1/11+..+1/46-1/51)

=>A.5= -2.(1-1/51)

=>5.A=-2.50/51

=>5.A=-100/51

=>A=-100/51:5

=>A=-20/51

    vậy -2/6+-2/66+-2/176+...+-2/46.51= -20/51

b, đặt B=8/3+8/15+8/35+..+8/323

=>B=8.(1/3+1/15+..+1/323)

=>2B=8.(2/1.3+2/3.15+...+2/17.19)

=>2B=8.(1-1/3+1/3-1/5+..+1/17-1/19)

=>2B=8.(1-1/19)

=>2B=8.18/19

=>2B=144/19

=>B=144/19:2

=>B=72/19

    vậy 8/3+8/15+8/35+..+8/323=72/19

8/3+8/15+8/35+....+8/323

=8/1.3+8/3.5+8/5.7+...+8/17.19

=2.4/1.3+2.4/3.5+2.4/5.7+...+2.4/17.19

=4.(2/1.3+2/3.5+2/5.7+...+2/17.19

=4.(1/1-1/3+1/3-1/5+1/5-1/7+...+1/17-1/19

=4.(1/1-1/19)

=4.18/19=72/19

THêm này

-2/6+-2/66+-2/176+...+-2/46.51

=-2/1.6+-2/6.11+-2/11.16+...+-2/46.51

=1/5.(-5/1.6+-5/6.11+-5/11.16+...+-5/46.51

=1/5.(-1/1-(-1/6)+(-1/6)-(-1/11)+(-1/11)-(-1/16)+...+(-1/46)-(-1/51)

=1/5.[-1/1-(-1/51)]

=1/5.-50/51

=-10/51

cái nầy do có dấu âm với dạng mới nên có j sai sót thì cmt nha

P/s: muốn bik 1/5 đâu ra đúng ko

đây làm chung tử là 5 vì hiệu dưới mẫu là 5 ( trong ngoặc)

định gửi link mà thôi khỏi :D