Đặt \(A=\frac{1}{30}+\frac{1}{42}+...+\frac{1}{90}+\frac{1}{110}\)
\(A=\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+\frac{1}{9.10}+\frac{1}{10.11}\)
\(A=\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}+\frac{1}{10}-\frac{1}{11}\)
\(A=\frac{1}{5}-\frac{1}{11}=\frac{11-5}{55}=\frac{6}{55}\)
Ủng hộ mk nha!!!
T = 1/30 + 1/42 + 1/56 + 1/72 + 1/90 + 1/110
T = 1/5.6 + 1/6.7 + 1/7.8 + 1/8.9 + 1/9.10 + 1/10.11
T = 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8 + 1/8 - 1/9 + 1/9 - 1/10 + 1/10 - 1/11
T = 1/5 - 1/11 (Cứ hai thằng cạnh nhau cộng lại bằng 0, chỉ còn thằng đầu và thằng cuối)
T = (11-5)/55
T = 14/55
1/30 + 1/42 + 1/ 56 + 1/72 + 1/90 + 1/110
= 1/5.6 + 1/6.7 + 1/7.8 + 1/8.9 + 1/9.10 + 1/10.11
= 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8 + 1/8 - 1/9 + 1/9 - 1/10 + 1/10 - 1/11
=1/5 - 1/11
= 11/55 - 5/55
= (11 - 5)/55
= 6/55
giúp thêm câu nữa:
B=2/35+4/77+2/143+4/221+2/323+4/437+2/575