Tính nhanh
1)A=\(\frac{7}{10.11}+\frac{7}{11.12}+\frac{7}{12.13}+...+\frac{7}{69.70}\)
2)B=\(\frac{1}{25.27}+\frac{1}{27.29}+\frac{1}{29.31}+...+\frac{1}{73.75}\)
3)C=\(\frac{4}{2.4}+\frac{4}{4.6}+\frac{4}{6.8}+...+\frac{4}{2008.2010}\)
tính tổng
a, A=\(\frac{7}{10.11}\)+\(\frac{7}{11.12}\)+\(\frac{7}{12.13}\)+.......+\(\frac{7}{69.70}\)
b, B=\(\frac{1}{25.27}\)+\(\frac{1}{27.29}\)+\(\frac{1}{29.31}\)+.........+\(\frac{1}{73.75}\)
c, C=\(\frac{4}{2.4}\)+\(\frac{4}{4.6}\)+\(\frac{4}{6.8}\)+.............+\(\frac{4}{2008.2010}\)
a,
suy ra A = 7. (1/10.11+1/11.12+1/12.13+.......+1/69.70)
suy ra A = 7. ( 1/10 - 1/11+ 1/11 - 1/12 + 1/12 - 1/13+ ............. + 1/69 - 1/70)
suy ra A = 7. ( 1/ 10 - 1/70)
suy ra A= 7. 3/35
suy ra A= 3/5
A=\(\frac{1}{7}\)x(\(\frac{1}{10}\)+\(\frac{1}{11}\)+\(\frac{1}{11}\)+\(\frac{1}{12}\)+\(\frac{1}{12}\)+\(\frac{1}{13}\)+......+\(\frac{1}{69}\)+\(\frac{1}{70}\))
A=\(\frac{1}{7}\)x(\(\frac{1}{10}\)+\(\frac{1}{70}\))
A=\(\frac{1}{7}\)x(\(\frac{7}{70}\)+\(\frac{1}{70}\))
A=\(\frac{1}{7}\)x\(\frac{4}{35}\)
A=\(\frac{4}{245}\)
1) A = 7/ 10.11 + 7/ 11.12 + 7/ 12.13 + .... + 7/69.70
2 ) B = 1/ 25.27 + 1/ 27.29 + 1/29.31+ ......+ 1/ 73.75
1) A=7/10.11+7/11.12+7/12.13+...+7/69.70
A=7.(1/10.11+1/11.12+1/12.13+...+1/69.70)
A= 7.(1/10-1/11+1/11-1/12+1/12-1/13+...+1/69-1/70)
A= 7.(1/10-1/70)
A=7.3/35=3/5
2)B=1/25.27+1/27.29+1/29.31+...+1/73.1/75
B=1/25-1/27+1/27-1/29+1/29-1/31+...+1/73-1/75
B=1/25-1/75=2/75
A = 7/ 10.11 + 7/ 11.12 + 7/ 12.13 + .... + 7/69.70
(1/7).A=1/10.11+1/11.12+...+1/69.70
=1/10-1/11+1/11-1/12+...+1/69-1/70
=1/10-1/70=3/35
=>A=7.(3/35)
=3/5
2 ) B = 1/ 25.27 + 1/ 27.29 + 1/29.31+ ......+ 1/ 73.75
=>(1/2).B=2/25.27+...+2.73.75
=1/25-1/27+...+1/73-1/75
=1/25-1/75
=2/75
=>B=4/75
A=7.(1/10.11+1/11.12+1/12.13+...+1/69.70)
A= 7.(1/10-1/11+1/11-1/12+1/12-1/13+...+1/69-1/70)
A= 7.(1/10-1/70)
A=7.3/35=3/5
2)B=1/25.27+1/27.29+1/29.31+...+1/73.1/75
B=1/25-1/27+1/27-1/29+1/29-1/31+...+1/73-1/75
B=1/25-1/75=2/75
A=7/10.11+7/11.12+7/12.13+...+7/69.70
B=1/25.27+1/27.29+1/29.31+...+1/73.75
\(A=\frac{7}{10.11}+\frac{7}{11.12}+\frac{7}{12.13}+...+\frac{7}{69.70}\)
\(B=\frac{1}{18}+\frac{1}{54}+\frac{1}{108}+...+\frac{1}{990}\)
\(C=\frac{1}{2}+\frac{5}{6}+\frac{11}{12}+\frac{19}{20}+...+\frac{89}{90}\)
A=.....
=\(7.\left(\frac{1}{10.11}+\frac{1}{11.12}+\frac{1}{12.13}+....+\frac{1}{69.70}\right)=7.\left(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+.....+\frac{1}{69}-\frac{1}{70}\right)\)
=\(7.\left(\frac{1}{10}-\frac{1}{70}\right)=7.\frac{3}{35}=\frac{3}{5}\)
MẤY PHẦN SAU CX TÁCH MẪU RA RÙI LÀM NHƯ VẬY
TỰ LÀM NHE
\(B=\frac{1}{3\cdot6}+\frac{1}{6\cdot9}+...+\frac{1}{30\cdot33}\)
\(B=\frac{1}{3}\cdot\left(\frac{3}{3\cdot6}+\frac{3}{6\cdot9}+...+\frac{3}{30\cdot33}\right)\)
\(B=\frac{1}{3}\cdot\left(\frac{1}{3}-\frac{1}{6}+\frac{1}{6}-\frac{1}{9}+...+\frac{1}{30}-\frac{1}{33}\right)\)
\(B=\frac{1}{3}\cdot\left(\frac{1}{3}-\frac{1}{33}\right)\)
\(B=\frac{1}{3}\cdot\frac{10}{33}=\frac{10}{99}\)
\(C=\left(1-\frac{1}{2}\right)+\left(1-\frac{1}{6}\right)+...+\left(1-\frac{1}{90}\right)\)
\(C=\left(1-\frac{1}{1\cdot2}\right)+\left(1-\frac{1}{2\cdot3}\right)+...+\left(1-\frac{1}{9\cdot10}\right)\)
\(C=9-\left(\frac{1}{1\cdot2}+\frac{1}{2\cdot3}+...+\frac{1}{9\cdot10}\right)\)
\(C=9-\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{9}-\frac{1}{10}\right)\)
\(C=9-\left(1-\frac{1}{10}\right)\)
\(C=9-\frac{9}{10}=\frac{81}{10}\)
các bạn đều sai hết mà các bạn lại cho những câu hỏi đó đúng
a) Tính:
\(\frac{7}{10.11}+\frac{7}{11.12}+\frac{7}{12.13}.......+\frac{7}{69.70}\)
= 7.( \(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+\frac{1}{12}-....-\frac{1}{70}\))
= 7.( \(\frac{1}{10}-\frac{1}{70}\))
= 7.(\(\frac{7}{70}-\frac{1}{70}\))
= 7.\(\frac{6}{70}\)
= \(\frac{3}{5}\)
Câu 1:Tìm x
\(\frac{x}{12}=\frac{3}{8}.\frac{7}{9};\)\(\frac{7}{x}=\frac{7}{12}.\frac{9}{2}\)
Câu 2:Tính nhanh
\(\frac{4}{25.27}+\frac{4}{27.29}+\frac{4}{29.31}+......+\frac{4}{49.51}\)
\(\frac{1}{25.27}+\frac{1}{27.29}+........+\frac{1}{49.51}\)
1) A=7/10.11+7/10.12+7/10.13+...+7/69.70
2) B=1/25.27+1/27.29+1/29.31+...+1/73.75
3) C=4/2.4+4/4.6+4/6.8+...+4/2008.2010
C=4/2.4+4/4.6+4/6.8+...+4/2008.2010
C = 2 ( 2 / 2.4 + 2/4.6 + 2/6.8 + ...+2/2008.2010)
C = 2 ( 1 - 1/4 + 1/4 - 1/6+1/6 - 1/8 +....+1/2008 - 1/2010 )
C = 2 ( 1 - 1 / 2010 )
C = 2 . 2009/2010
C = 2009 / 1005
Chúc bạn học tốt !
bạn tách ra từng bài một mình sẽ giúp
B = \(\frac{1}{25.27}\)+ \(\frac{1}{27.29}+\frac{1}{29.31}+...+\frac{1}{73.75}\)
2B = 2 ( 1/25.27+1/27.29+1/29.31+...+1/73.75) Mình x 2 lên tí phải chia cho 2
2B = \(\frac{2}{25.27}+\frac{2}{27.29}+...+\frac{2}{73.75}\)
2B = 1/25 - 1/27+ 1/27-1/29 + .... + 1/73 - 1/75
2B = 1/25 - 1/75
2B = 3/75 - 1/75
2B = 2/75
=> B = 2/75 : 2
B = 2/75 . 1/2
B = 1/75
Chúc bạn học tốt
bài 1:
a)A=\(\frac{6}{10.11}+\frac{6}{11.12}+\frac{6}{12.13}+...+\frac{6}{69.70}\)
b)B=\(\frac{4}{2.4}+\frac{4}{4.6}+\frac{4}{6.8}+...+\frac{4}{2018.2020}\)
giúp mk với mk đang cần gấp
ai nhanh mk tick cho
a, \(A=\frac{6}{10.11}+\frac{6}{11.12}+\frac{6}{12.13}+...+\frac{6}{69.70}\)
\(A=\frac{6}{10}-\frac{6}{11}+\frac{6}{11}-\frac{6}{12}+\frac{6}{12}-\frac{6}{13}+...+\frac{6}{69}-\frac{6}{70}\)
\(A=\frac{6}{10}-\frac{6}{70}\)
\(A=\frac{18}{35}\)
b, \(B=\frac{4}{2.4}+\frac{4}{4.6}+\frac{4}{6.8}+...+\frac{4}{2018.2020}\)
\(B=\frac{4}{2}.\left(\frac{2}{2.4}+\frac{2}{4.6}+\frac{2}{6.8}+...+\frac{2}{2018.2020}\right)\)
\(B=2.\left(\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+\frac{1}{6}-\frac{1}{8}+...+\frac{1}{2018}-\frac{1}{2020}\right)\)
\(B=2.\left(\frac{1}{2}-\frac{1}{2020}\right)\)
\(B=2.\frac{1009}{2020}\)
\(B=\frac{1009}{1010}\)
Chúc bạn học tốt
Tính tổng dãy số cáh đều( tính nhanh)
a, \(\frac{7}{10.11}\)+ \(\frac{7}{11.12}\)+.....+\(\frac{7}{69.70}\)
b, \(\frac{6}{15.18}\)+ \(\frac{6}{18.21}\)+......+\(\frac{6}{87.60}\)
c, \(\frac{1}{25.27}\)+ \(\frac{1}{27.29}\)+.......+\(\frac{1}{73.75}\)
d, \(\frac{1}{1.2.3}\)+ \(\frac{1}{2.3.4}\)+ \(\frac{1}{3.4.5}\)+.....+\(\frac{1}{18.19.20}\)
Làm chi tiết hộ mình nha