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a) 3/7 + 4/9 + 4/7 + 5/9
= ( 3/7 + 4/7 ) + ( 4/9 + 5/9 )
= 7/7 + 9/9
= 1 + 1
= 2
b)1/5 + 4/10 + 9/15 + 16/20 + 25/25 + 36/30 + 49/35 + 64/40 + 81/45
= 1/5 + 2/5 + 3/5 + 4/5 + 5/5 + 6/5 + 7/5 + 8/5 + 9/5
= ( 1/5 + 9/5 ) + ( 2/5 + 8/5 ) + (7/5 + 3/5 ) + ( 4/5 + 6/5 ) + 5/5
= 2 + 2 + 2 + 2 + 1
= 2 x 4 + 1
= 8 +1
= 9
c) 1/8 + 1/12 + 3/8 + 5/12
= ( 1/8 + 3/8 ) + ( 1/12 + 5/12)
= 4/8 + 6/12
= 1/2 + 1/2
= 2/4 = 1/2
mỏi tay rồi
A = ( -4/5 + 4/3 ) + (-5/4 + 14/5) - 7/3
= 8/15 + 31/20 - 7/3
= 25/12 - 7/3
= -1/4
B = 8/3 x 2/5 x 3/8 x 10x 19/92
= 16/15 x 15/4 x 19/92
= 4x19/92
= 19/23
toi nay mk se lay y kien cua cac ban , giup mk nha
b1
a) \(\dfrac{1}{5.6}+\dfrac{1}{6.7}+\dfrac{1}{7.8}+\dfrac{1}{8.9}+\dfrac{1}{9.10}\)
\(=\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{10}\)
\(=\dfrac{1}{5}-\dfrac{1}{10}\)
\(=\dfrac{2}{10}-\dfrac{1}{10}\)
\(=\dfrac{1}{10}\)
b) \(\dfrac{1}{1.2}+\dfrac{1}{2.3}+...+\dfrac{1}{99.100}\)
\(=\dfrac{1}{1}-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+...+\dfrac{1}{99}-\dfrac{1}{100}\)
\(=\dfrac{1}{1}-\dfrac{1}{100}\)
\(=\dfrac{99}{100}\)
c) \(\dfrac{2}{3.5}+\dfrac{2}{5.7}+\dfrac{2}{7.9}+\dfrac{2}{9.11}\)
\(=\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{11}\)
\(=\dfrac{1}{3}-\dfrac{1}{11}\)
\(=\dfrac{8}{33}\)
d) \(\dfrac{2}{3.5}+\dfrac{2}{5.7}+...+\dfrac{2}{99.101}\)
\(=\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+...+\dfrac{1}{99}-\dfrac{1}{101}\)
\(=\dfrac{1}{3}-\dfrac{1}{101}\)
\(=\dfrac{98}{303}\)
a: \(\dfrac{11}{4}+\left(-\dfrac{3}{5}+\dfrac{1}{3}\right):\dfrac{-4}{9}\)
\(=\dfrac{11}{4}+\dfrac{-9+5}{15}\cdot\dfrac{9}{-4}\)
\(=\dfrac{11}{4}+\dfrac{-4}{15}\cdot\dfrac{9}{-4}\)
\(=\dfrac{11}{4}+\dfrac{3}{5}=\dfrac{55+12}{20}=\dfrac{67}{20}\)
b: \(\dfrac{2}{6}-\left(-\dfrac{3}{3}+\dfrac{4}{7}\right):\dfrac{-9}{5}\)
\(=\dfrac{2}{6}-\left(-1+\dfrac{4}{7}\right)\cdot\dfrac{-5}{9}\)
\(=\dfrac{2}{6}-\dfrac{-3}{7}\cdot\dfrac{-5}{9}\)
\(=\dfrac{1}{3}-\dfrac{15}{63}\)
\(=\dfrac{6}{63}=\dfrac{2}{21}\)
c: \(21-\dfrac{15}{4}:\left(\dfrac{3}{8}-\dfrac{1}{6}\right)\)
\(=21-\dfrac{15}{4}:\dfrac{3\cdot3-4}{24}\)
\(=21-\dfrac{15}{4}\cdot\dfrac{24}{5}\)
\(=21-3\cdot6=3\)
\(a.\dfrac{11}{4}+\left(-\dfrac{3}{5}+\dfrac{1}{3}\right):-\dfrac{4}{9}\)
\(=\dfrac{11}{4}+\left(-\dfrac{4}{15}\right):-\dfrac{4}{9}\)
\(=\dfrac{11}{4}+\dfrac{3}{5}=\dfrac{67}{20}\)
\(b.\dfrac{2}{6}-\left(-\dfrac{3}{3}+\dfrac{4}{7}\right):-\dfrac{9}{5}\)
\(=\dfrac{2}{6}-\left(-\dfrac{3}{7}\right):-\dfrac{9}{5}\)
\(=\dfrac{2}{6}-\dfrac{5}{21}=\dfrac{2}{21}\)
\(c.21-\dfrac{15}{4}:\left(\dfrac{3}{8}-\dfrac{1}{6}\right)\)
\(=21-\dfrac{15}{4}:\dfrac{5}{24}\)
\(=21-18=3\)
M = 5 + 53 + 55 + ... + 547 + 549
52M = 52(5 + 53 + 55 + ... + 547 + 549)
25M = 53 + 55 + 57 + ... + 549 + 551
25M - M = ( 53 + 55 + 57 + ... + 549 + 551) - (5 + 53 + 55 + ... + 547 + 549)
24M = 551 - 5
M = \(\frac{5^{51}-5}{24}\)
Giải:
A=5/9+2/15-6/9
=(5/9-6/9)+2/15
= -1/9 + 2/15
= 1/45
B=2/7-3/8+4/7+1/7-5/8+5/15
= (2/7+4/7+1/7) + (-3/8-5/8) +1/3
= 1+ (-1) +1/3
=1/3
C=3/5+1/15+1/57+1/3-2/9-3/4-1/36
=9/15+1/15+1/57+19/57-8/36-27/36-1/36
=(9/15+1/15)+(1/57+19/57)+(-8/36-27/36-1/36)
=2/3+20/57+(-1)
=58/57+(-1)
=1/57
D=1/1.2+1/2.3+1/3.4+...+1/99.100
=1/1-1/2+1/2-1/3+1/3-1/4+...+1/99-1/100
=1/1-1/100
=99/100
Câu E mình ko biết làm nhé!