Tính nhanh: 2 3.5 + 3 5.8 + 11 8.19 + 13 19.32 + 25 32.57 + 30 57.87
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BN
12 tháng 2 2017
\(\frac{2}{3.5}+\frac{3}{5.8}+\frac{11}{8.19}+\frac{13}{19.32}+\frac{25}{32.57}+\frac{30}{57.87}\)
\(=\frac{5-3}{3.5}+\frac{8-5}{3}+\frac{19-8}{8.19}+\frac{32-29}{19.32}+\frac{57-32}{32.57}+\frac{87-57}{57.87}\)
\(=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{19}+\frac{1}{19}-\frac{1}{32}+\frac{1}{32}-\frac{1}{57}+\frac{1}{57}-\frac{1}{87}\)
\(=\frac{1}{3}-\frac{1}{87}=\frac{28}{87}\)
DV
2
MT
19 tháng 10 2015
\(A=\frac{2}{3.5}+\frac{3}{5.8}+\frac{11}{8.19}+\frac{13}{19.32}+\frac{25}{32.57}+\frac{30}{57.87}\)
\(=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{19}+\frac{1}{19}-\frac{1}{32}+\frac{1}{32}-\frac{1}{57}+\frac{1}{57}-\frac{1}{87}\)
\(=\frac{1}{3}-\frac{1}{87}=\frac{29}{87}-\frac{1}{87}=\frac{28}{87}\)
NT
1
LM
1 tháng 3 2017
\(C=\dfrac{2}{3\cdot5}+\dfrac{3}{5\cdot8}+\dfrac{11}{8\cdot19}+\dfrac{13}{19\cdot32}+\dfrac{25}{32\cdot57}+\dfrac{30}{57\cdot87}\)\(C=\left(\dfrac{5-3}{3\cdot5}\right)+\left(\dfrac{8-5}{5\cdot8}\right)+\left(\dfrac{19-8}{8\cdot19}\right)+\left(\dfrac{32-19}{19\cdot32}\right)+\left(\dfrac{57-32}{32\cdot57}\right)+\left(\dfrac{87-57}{57\cdot87}\right)\)\(C=\left(\dfrac{1}{3}-\dfrac{1}{5}\right)+\left(\dfrac{1}{5}-\dfrac{1}{8}\right)+\left(\dfrac{1}{8}-\dfrac{1}{19}\right)+\left(\dfrac{1}{19}-\dfrac{1}{32}\right)+\left(\dfrac{1}{32}-\dfrac{1}{57}\right)+\left(\dfrac{1}{57}+\dfrac{1}{87}\right)\)\(C=\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{19}+\dfrac{1}{19}-\dfrac{1}{32}+\dfrac{1}{32}-\dfrac{1}{57}+\dfrac{1}{57}-\dfrac{1}{87}\)\(C=\dfrac{1}{3}-\dfrac{1}{87}=\dfrac{28}{87}\)
VB
1
20 tháng 3 2019
\(S=\frac{2}{3.5}+\frac{3}{5.8}+\frac{11}{8.19}+\frac{13}{19.32}+\frac{25}{32.57}+\frac{30}{57.85}\)
\(S=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{19}+\frac{1}{19}-\frac{1}{32}+\frac{1}{32}-\frac{1}{57}+\frac{1}{57}-\frac{1}{87}\)
\(S=\frac{1}{3}-\frac{1}{87}\)
\(S=\frac{28}{87}\)
Y
3 tháng 3 2019
\(A=\dfrac{1}{2}+\dfrac{1}{6}+\dfrac{1}{12}+...+\dfrac{1}{9900}\)
\(A=\dfrac{1}{1\cdot2}+\dfrac{1}{2\cdot3}+\dfrac{1}{3\cdot4}+...+\dfrac{1}{99\cdot100}\)
\(A=1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{99}-\dfrac{1}{100}\)
\(A=1-\dfrac{1}{100}=\dfrac{99}{100}\)
\(B=\dfrac{1}{3}+\dfrac{1}{15}+\dfrac{1}{35}+..+\dfrac{1}{195}\) ( là 195 ms đúng ! )
\(B=\dfrac{1}{1\cdot3}+\dfrac{1}{3\cdot5}+\dfrac{1}{5\cdot7}+...+\dfrac{1}{13\cdot15}\)
\(B=\dfrac{1}{2}\left(1-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+...+\dfrac{1}{13}-\dfrac{1}{15}\right)\)
\(B=\dfrac{1}{2}\left(1-\dfrac{1}{15}\right)=\dfrac{1}{2}\cdot\dfrac{14}{15}=\dfrac{7}{15}\)
\(C=\dfrac{1}{2\cdot4}+\dfrac{1}{4\cdot6}+\dfrac{1}{6\cdot8}+...+\dfrac{1}{98\cdot100}\)
Rồi làm tương tự cân b nha!
\(D=\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{19}+\dfrac{1}{19}-\dfrac{1}{32}+\dfrac{1}{32}-\dfrac{1}{57}\)
\(+\dfrac{1}{57}-\dfrac{1}{87}\)
\(D=\dfrac{1}{3}-\dfrac{1}{87}=\dfrac{28}{87}\)
NC
1
L
27 tháng 3 2021
\(D=\dfrac{2}{3}.\dfrac{5}{5.8}+\dfrac{11}{8.19}+\dfrac{13}{19.32}\)
\(D=\dfrac{2.5}{2.5.8}+\dfrac{1}{19}.\dfrac{11}{8}+\dfrac{1}{19}.\dfrac{13}{32}\)
\(D=\dfrac{1}{12}+\dfrac{1}{19}.\left(\dfrac{11}{8}+\dfrac{13}{32}\right)\)
\(D=\dfrac{1}{12}+\dfrac{1}{19}.\left(\dfrac{44}{32}+\dfrac{13}{32}\right)\)
\(D=\dfrac{1}{12}+\dfrac{1}{19}.\dfrac{57}{32}\)
\(D=\dfrac{1}{12}+\dfrac{3}{32}\)
\(D=\dfrac{8}{96}+\dfrac{9}{96}\)
\(D=\dfrac{17}{96}\)
NC
1
27 tháng 3 2021
\(D=\dfrac{2}{3}.\dfrac{5}{5.8}+\dfrac{11}{8.19}+\dfrac{13}{19.32}\)
\(=\dfrac{2.5}{3.(5.8)}+\dfrac{11}{8.19}+\dfrac{13}{19.32}\)
\(=\dfrac{10}{3.5.8}+\dfrac{11}{8.19}+\dfrac{13}{19.32}\)
\(=\dfrac{1}{3.4}+\dfrac{11}{8.19}+\dfrac{13}{19.32}\)
\(=\dfrac{17}{96}\)
TQ
3
20 tháng 3 2019
Ai chs opoke đại chiên lh mik nha! Đỏi lấy nick olm hoặc cho mik
2 3.5 + 3 5.8 + 11 8.19 + 13 19.32 + 25 32.57 + 30 57.87 = 1 3 − 1 5 + 1 5 − 1 8 + 1 8 − 1 19 + 1 19 − 1 32 + 1 32 − 1 57 + 1 57 − 1 87 = 1 3 − 1 87 = 29 87 − 1 87 = 28 87