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a)\(A=\frac{2}{3}+\frac{2}{6}+\frac{2}{12}+\frac{2}{24}+\frac{2}{48}+\frac{2}{96}+\frac{2}{192}\)
\(\frac{1}{2}xA=\frac{1}{3}+\frac{1}{6}+\frac{1}{12}+\frac{1}{24}+\frac{1}{48}+\frac{1}{96}+\frac{1}{192}\)
\(\frac{1}{4}xA=\frac{1}{6}+\frac{1}{12}+\frac{1}{24}+\frac{1}{48}+\frac{1}{96}+\frac{1}{192}+\frac{1}{384}\)
\(\frac{1}{4}xA-\frac{1}{2}xA=\frac{1}{3}-\frac{1}{384}\)
\(\frac{1}{4}xA=\frac{127}{384}\)
\(A=\frac{127}{384}:\frac{1}{4}\)
\(A=\frac{127}{96}\)
\(\frac{1}{2}+\frac{5}{6}+\frac{11}{12}+\frac{19}{20}+...+\frac{89}{90}\)
\(=1-\frac{1}{2}+1-\frac{1}{6}+1-\frac{1}{12}+1-\frac{1}{20}+...+1-\frac{1}{90}\)
\(=9-\left(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{90}\right)\)
\(=9-\left(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{9.10}\right)\)
\(=9-\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+....+\frac{1}{9}-\frac{1}{10}\right)\)
\(=9-\left(1-\frac{1}{10}\right)\)
\(=9-\frac{9}{10}=\frac{81}{10}\)
a )Tổng của dãy số trên là
( 0,19+0,1)*19 /2 =2,755
Đáp số ; 2,755
mình không có thời gian lam câu b
Bài 1:
a; (\(\dfrac{1}{4}\)\(x\) - \(\dfrac{1}{8}\)) x \(\dfrac{3}{4}\) = \(\dfrac{1}{4}\)
\(\dfrac{1}{4}x\) - \(\dfrac{1}{8}\) = \(\dfrac{1}{4}\) : \(\dfrac{3}{4}\)
\(\dfrac{1}{4}\)\(x\) - \(\dfrac{1}{8}\) = \(\dfrac{1}{4}\) x \(\dfrac{4}{3}\)
\(\dfrac{1}{4}x\) - \(\dfrac{1}{8}\) = \(\dfrac{1}{3}\)
\(\dfrac{1}{4}x\) = \(\dfrac{1}{3}\) + \(\dfrac{1}{8}\)
\(\dfrac{1}{4}\) \(x\)= \(\dfrac{8}{24}\) + \(\dfrac{11}{24}\)
\(\dfrac{1}{4}x=\dfrac{11}{24}\)
\(x=\dfrac{11}{24}:\dfrac{1}{4}\)
\(x=\dfrac{11}{24}\times4\)
\(x=\dfrac{11}{6}\)
b; \(\dfrac{12}{5}:x\) = \(\dfrac{14}{3}\) x \(\dfrac{4}{7}\)
\(\dfrac{12}{5}\) : \(x\) = \(\dfrac{8}{3}\)
\(x\) = \(\dfrac{12}{5}\) : \(\dfrac{8}{3}\)
\(x\) = \(\dfrac{12}{5}\) x \(\dfrac{3}{8}\)
\(x\) = \(\dfrac{9}{10}\)
Bài 1:
a) [ (1/6 + 1/10 + 1/15) : (1/6 + 1/10 - 1/15) phần 1/2 - 1/3 + 1/4 - 1/5 ] : (1/4 - 1/6)
= [ (1/6 : 1/6) + (1/10 : 1/10) - (1/15 : 1/15) phần 30/60 - 20/60 + 15/60 - 12/60 ] : (3/12 - 2/12)
= [ 1 + 1 - 1 phần 13/60 ] : 1/12
= [ 1 : 13/60 ] x 12
= 60/13 x 12
=720/ 13
b) (3/20 + 1/2 - 1/15) x 12/49 phần 3 và 1/3 + 2/9
= (9/60 + 30/60 - 4/60) x 12/49 phần 10/3 + 2/9
= 7/12 x 12/49 phần 30/9 + 2/9
= 1/7 : 32/9
= 1/7 x 9/32
= 9/224
A=126+72+163
A=397
B=1/6 x 1,25+0,25
B=1/6x5/4+1/4
B=5/24+1/4
B=11/24
c ko tìm đc
a) 0,1 + 0,2 + 0,3 + 0,4 + ... + 1,9
= ( 1,9 + 0,1 ) x 19 : 2
= 2 x 19 : 2
= 19
b) (1999 x 1998 + 1998 + 1997) x ( 1 + 1/2 : 1 1/2 - 1 1/3)
= (1999 x 1998 + 1998 + 1997) x 0
= 0
a) A = 2 + 4 + 6 + 8 + ... + 1000
Ta có : A = 2 + 4 + 6 + 8 + ... + 1000 ( có 500 số )
= (1000 + 2) . 500 : 2 = 250500
c) \(\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{97.99}+\frac{2}{99.101}\)
\(=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{97}-\frac{1}{99}+\frac{1}{99}-\frac{1}{101}\)
\(=1-\frac{1}{101}=\frac{100}{101}\)
Câu 2:
a; \(x\) x \(\dfrac{1}{6}\) = \(\dfrac{2}{3}\) + \(\dfrac{3}{4}\)
\(x\) x \(\dfrac{1}{6}\) = \(\dfrac{17}{12}\)
\(x\) = \(\dfrac{17}{12}\) : \(\dfrac{1}{6}\)
\(x\) = \(\dfrac{17}{2}\)
b; 72 : 24 x \(x\) = 345
3 x \(x\) = 345
\(x\) = 345 : 3
\(x=\) 115
c; 2 x (\(x-3\)) + 1,5 x 0,3 = 19
2 x (\(x-3\)) + 0,45 = 19
2 x (\(x-3\)) = 19 - 0,45
2 x (\(x-3\)) = 18,55
\(x-3\) = 18,55 : 2
\(x-3\) = 9,275
\(x\) = 9,275 + 3
\(x=\) 12,275
Câu 1:
20 - 4,9 : 0,7 + 1,2 x 5
= 20 - 7 + 6
= 13 + 6
= 19