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\(A=\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}\)
\(A=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}\)
\(A=\frac{1}{2}-\frac{1}{8}\)
\(A=\frac{3}{8}\)
A=\(\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{7.8}\)
A=\(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+..._{ }+\frac{1}{7}-\frac{1}{8}\)
A=\(\frac{1}{2}-\frac{1}{8}\)
A=\(\frac{3}{8}\)
a/-13/24
b/15/14
c/ngu thì bấm cho nhiều vào
d/2.75
bấm tìm X đi ngu quá bấm nhiều vô
1/ \(\frac{1}{1.3}+\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+\frac{1}{9.11}+\frac{1}{11.13}\)
\(=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}+\frac{1}{11}-\frac{1}{13}\)
\(=1-\frac{1}{13}=\frac{12}{13}\)
\(\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+.............+\frac{1}{66}\)
\(=\frac{2}{20}+\frac{2}{30}+\frac{2}{42}+..............+\frac{2}{132}\)
\(=2.\left(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+...........+\frac{1}{11.12}\right)\)
\(=2.\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...........+\frac{1}{11}-\frac{1}{12}\right)\)
\(=2.\left(\frac{1}{4}-\frac{1}{12}\right)\)
\(=2.\frac{1}{6}\)
\(=\frac{1}{3}\)
Sai đề rồi
Khong phải là ..... ... . . .. . .
mà là +++++++++++
a) = 1/5.6 + 1/6.7 + 1/7.8 + .. + 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
= 6/55
b) 2/35 + 2/63 + 2/99 + .. + 2/255
= 2/5.7 + 2/7.9 + 2/9.11 + 2/11.13 + 2/13.15 + 2/15.17
= 1/5 - 1/7 + 1/7 - 1/9 + 1/9 - 1/11 + 1/11 - 1/13 + 1/13 - 1/15 + 1/15 - 1/17
= 1/5 - 1/17
= 12/85
-1/4.5 - 1/5.6 - 1/6.7 - 1/7.8 - 1/8.9 - 1/9/10
= -1/4 - (-1/5)+......+(-1/9)-(-1/10)
= -1/4 - (-1/10)
= -1/4 + 1/10
= -3/20
hoang tu bd sai rui phai dat dau am ra ngoai rui ms tinh