Đáp án:
 a,1/5 x 6 +1/6 x 7+..........+1/ 24 x 25
=1/5-1/6+1/6-1/7+......+1/24-1/25
=1/5-1/25=5/25-1/25=4/25
b, 1/10+1/15+1/21+1/28+1/36+1/45
=1/2. (1/20+1/30+1/42+1/56+1/72+1/90)
=1/2.(1/4 x 5+1/5 x 6+....+1/ 9 x 10)
=1/2.(1/4-1/5+1/5-1/6+........+1/9-1/10)
=1/2.(1/4-1/10)=1/2.(5/20-2/20)=1/2.3/20=3/40
XIN HAY NHẤT NHA