bài 1 : Tính
1) \(\left(\frac{1}{2}+1\right).\left(\frac{1}{3}+1\right).\left(\frac{1}{4}+1\right).....\left(\frac{1}{99}+1\right)\)
2) \(\left(\frac{1}{2}-1\right).\left(\frac{1}{3}-1\right).\left(\frac{1}{4}-1\right).....\left(\frac{1}{99}-1\right)\)
3) \(\frac{3}{2^2}.\frac{8}{3^2}.\frac{15}{4^2}...........\frac{899}{30^2}\)
4) \(\frac{4}{3.7}+\frac{4}{7.11}+\frac{4}{11.15}+......+\frac{4}{107.111}\)
5) \(\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+.........+\frac{2}{399}\)
6) \(\frac{7}{10.11}+\frac{7}{11.12}+\frac{7}{12.13}+....+\frac{7}{69.70}\)
7) \(\frac{6}{15.18}+\frac{6}{18.21}+\frac{6}{21.24}+.....+\frac{6}{87.90}\)
1) =3/2 . 4/3 . 5/4 ...... 100/99 = 100/2 = 50
2) = -1/2 . -2/3 .-3/4 ..... . -98/99 = 1/99 (Tích này có 98 thừa số âm, 98 là số chẵn nên tích mang dấu dương)