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A

\(\frac{1}{1+2}+\frac{1}{2+3}+\frac{1}{3+4}+...+\frac{1}{13+14}=\frac{1}{2.3:2}+\frac{1}{3.4:2}+\frac{1}{4.5:2}+...+\frac{1}{14.15:2}=\frac{2}{2.3}+\frac{2}{3.4}+\frac{2}{4.5}+...+\frac{2}{14.15}\)\(=2\left(\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{14.15}\right)=2\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{14}-\frac{1}{15}\right)\)

\(=2\left(\frac{1}{2}-\frac{1}{15}\right)=\frac{1}{2}.2-\frac{1}{15}.2=1-\frac{2}{15}=\frac{13}{15}\)

2cm=0,02m

20cm=0,2m

Thể tích hình trụ là:0,02².0,2.3,14=157/625000m³

^{m=D.V=2700.157/625000 =0,67824kg =6,7824N}