\(C=\frac{3}{2.5}+\frac{3}{5.8}+\frac{3}{8.11}+....+\frac{3}{65.68}\)

\(C=\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+....+\frac{1}{65}-\frac{1}{68}\)

\(C=\frac{1}{2}-\frac{1}{68}=\frac{33}{68}\)

Câu D bạn làm tương tự .

\(E=\frac{5}{6.10}+\frac{5}{10.14}+...+\frac{5}{86.90}\)

\(E=\frac{5}{4}.\left(\frac{4}{6.10}+\frac{4}{10.14}+....+\frac{4}{86.90}\right)\)

\(E=\frac{5}{4}.\left(\frac{1}{6}-\frac{1}{10}+\frac{1}{10}-\frac{1}{14}+...+\frac{1}{86}-\frac{1}{90}\right)\)

\(E=\frac{5}{4}.\left(\frac{1}{6}-\frac{1}{90}\right)\)

\(E=\frac{5}{4}.\frac{7}{45}=\frac{7}{36}\)

Câu F bạn đặt 3/5 ra ngoài rồi làm tương tự câu E

Mk cảm ơn

Giải :

Ta có :

\(A=\frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{119.120}\)

\(\Rightarrow A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{119}-\frac{1}{120}\)

\(\Rightarrow A=1-\frac{1}{120}=\frac{119}{120}\)

\(B=\frac{1}{10.11}+\frac{1}{11.12}+...+\frac{1}{999.1000}\)

\(\Rightarrow B=\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+...+\frac{1}{999}-\frac{1}{1000}\)

\(\Rightarrow B=\frac{1}{10}-\frac{1}{1000}=\frac{99}{1000}\)

\(A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{119.120}.\)

\(A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{119}-\frac{1}{120}\)

\(A=1-\frac{1}{120}=\frac{119}{120}\)

\(B=\frac{1}{10.11}+\frac{1}{11.12}+\frac{1}{12.13}+...+\frac{1}{999.1000}\)

\(B=\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+\frac{1}{12}-\frac{1}{13}+...+\frac{1}{999}-\frac{1}{1000}\)

\(B=\frac{1}{10}-\frac{1}{1000}\)

\(B=\frac{99}{1000}\)

1/2- lxl = -1/3

=> lxl  = 1/2 + 1/3

=> lxl  = 5/6

=> |x| = 1/2 + 1.3

=> |x| = 1/5

=> x = -1/5 hoặc 1/5

hok tốt

IKIHJHJJJN .?

\(-\frac{9}{4}-\frac{5}{8}\div x=\frac{3}{7}\)

\(\frac{5}{8}\div x=-\frac{9}{4}-\frac{3}{7}\)

\(\frac{5}{8}\div x=-\frac{75}{28}\)

\(x=\frac{5}{8}\div\left(-\frac{75}{28}\right)\)

\(x=-\frac{7}{30}\)

\(-\frac{5}{3}+\frac{5}{6}\cdot x=5\)

\(\Rightarrow\)\(\frac{5}{6}x=5+\frac{5}{3}=\frac{20}{3}\)

\(\Rightarrow\)\(x=\frac{20}{3}:\frac{5}{6}\)

\(\Rightarrow\)\(x=\frac{120}{15}=8\)

Mình chỉ viết kết quả,ko diễn giải nhé:

x=8 nhé!

hình như thiếu đề thì phải