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a)\(\dfrac{5}{7}+\dfrac{4}{9}=\dfrac{45}{63}+\dfrac{28}{63}=\dfrac{73}{63}\) ; \(\dfrac{9}{11}+\dfrac{3}{8}=\dfrac{72}{88}+\dfrac{33}{88}=\dfrac{105}{88}\)
\(\dfrac{4}{5}-\dfrac{2}{3}=\dfrac{12}{15}-\dfrac{10}{15}=\dfrac{2}{15}\); \(\dfrac{16}{25}-\dfrac{2}{5}=\dfrac{16}{25}-\dfrac{10}{25}=\dfrac{6}{25}\)
b)\(5+\dfrac{3}{5}=\dfrac{25}{5}+\dfrac{3}{5}=\dfrac{28}{5};10-\dfrac{9}{16}=\dfrac{160}{16}-\dfrac{9}{16}=\dfrac{151}{16}\)
\(\dfrac{2}{3}-\left(\dfrac{1}{6}+\dfrac{1}{8}\right)=\dfrac{2}{3}-\left(\dfrac{8}{48}+\dfrac{6}{48}\right)=\dfrac{2}{3}-\dfrac{14}{48}=\dfrac{32}{48}-\dfrac{14}{48}=\dfrac{3}{8}\)
a, \(\frac{2}{3}+\frac{2}{3}+\frac{6}{3}=\frac{10}{3}\)
b,\(\frac{3}{4}+\frac{3}{4}+\frac{3}{2}=\frac{6}{4}+\frac{3}{2}=\frac{3}{2}+\frac{3}{2}=\frac{6}{2}=3\)
b) 1/3+1/3^2+1/3^3+1/3^4+1/3^5 (goi tong bang M)
3M=1+1/3+1/3^2+1/3^3+1/3^4
3M-M=1-1/3^5
2M=242/243
M=242/243*1/2=121/243
Bài 1 :
A = 1 - 1/2 + 1/2 - 1/4 + 1/4 - 1/8 + 1/8 - 1/16 + ... + 1/32 - 1/64
A = 1 - 1/64
A = 63/64
12/5 -4/5 =8/5
11/6 -2/3 =21/18 =7/6
7/8 -2/7 =33/56
4 -8/5 =20/5-8/5 =12/5
2 -3/8 =16/8 -3/8 =13/8
16/7 -2 =16/7 -14/7 =2/7
25/4 -3=25/4 -12/4 =13/4
câu 2 : câu 3 :
a) 8/5 a)4 - 8/5 = 20/5 - 8/5 =12/5
b) 7/6 b)2 - 3/8 =16/8 - 3/8 = 13/8
c) 35/56 c)16/7 - 2 = 16/7 - 14/7 = 2/7
d)25/4 - 3 = 25/4 - 12/4 = 13/4
\(a,\frac{3}{15}+\frac{2}{5}=\frac{1}{5}+\frac{2}{5}=\frac{3}{5}.\)
\(b,\frac{9}{8}-\frac{5}{6}=\frac{7}{24}\)
\(c,\frac{1}{2}+\frac{3}{7}+\frac{11}{14}=\frac{13}{14}+\frac{11}{14}=\frac{24}{14}=\frac{12}{7}\)
\(d,\frac{8}{3}-\frac{1}{2}-1=\frac{13}{6}-1=\frac{7}{6}\)
~~~ học tốt ~~~
Sao mà mình hỏi bài này từ lâu lắm rồi mà vẫn chưa có bạn nào trả lời nhỉ?
A) \(\dfrac{1}{2}+\dfrac{1}{4}+\dfrac{1}{8}+\dfrac{1}{16}+\dfrac{1}{32}+\dfrac{1}{64}\)
2A= \(1+\dfrac{1}{2}+\dfrac{1}{4}+\dfrac{1}{8}+\dfrac{1}{16}+\dfrac{1}{32}\)
2A-A = \(1-\dfrac{1}{32}\)
A= \(\dfrac{31}{32}\)
b)\(\dfrac{1}{3}+\dfrac{1}{9}+\dfrac{1}{27}+\dfrac{1}{81}+\dfrac{1}{243}+\dfrac{1}{729}\)
Dặt A=
3A= \(1+\dfrac{1}{3}+\dfrac{1}{9}+\dfrac{1}{27}+\dfrac{1}{81}+\dfrac{1}{243}\)
3A-A=\(1-\dfrac{1}{792}\)
2A= \(\dfrac{791}{792}\)
A= \(\dfrac{791}{792}:2=\dfrac{791}{1584}\)
c)\(\dfrac{1}{5}+\dfrac{1}{10}+...+\dfrac{1}{1280}\)
Đặt A= \(\dfrac{1}{5}+\dfrac{1}{10}+...+\dfrac{1}{1280}\)
\(2A=\dfrac{2}{5}+\dfrac{1}{5}+\dfrac{1}{10}+....+\dfrac{1}{640}\)
\(2A-A=\dfrac{2}{5}-\dfrac{1}{1280}\)
\(A=\dfrac{511}{1280}\)
d) \(\dfrac{2}{3}+\dfrac{2}{6}+\dfrac{2}{12}+...+\dfrac{2}{192}\)
Đặt A= \(\dfrac{2}{3}+\dfrac{2}{6}+\dfrac{2}{12}+...+\dfrac{2}{192}\)
\(2A=\dfrac{4}{3}+\dfrac{2}{3}+\dfrac{2}{6}+...+\dfrac{2}{192}\)
\(2A-A=\dfrac{4}{3}-\dfrac{2}{192}\)
\(A=\dfrac{127}{96}\)
e)\(1+\dfrac{5}{4}+\dfrac{5}{8}+\dfrac{5}{16}+\dfrac{5}{32}+\dfrac{5}{64}\)
Đặt A = \(1+(\dfrac{5}{4}+\dfrac{5}{8}+\dfrac{5}{16}+\dfrac{5}{32}+\dfrac{5}{64})\)
2A= 1+ \((\dfrac{5}{2}+\dfrac{5}{4}+\dfrac{5}{8}+\dfrac{5}{16}+\dfrac{5}{32})\)
2A-A = 1+ (\(\dfrac{5}{2}-\dfrac{5}{32}\))
A= 1 + \(\dfrac{75}{32}\)
A= \(\dfrac{107}{32}\)
f) \(\dfrac{3}{2}+\dfrac{3}{8}+\dfrac{3}{32}+\dfrac{3}{128}+\dfrac{3}{512}\)
Đặt B =
4B= \(6+\dfrac{3}{2}+\dfrac{3}{8}+\dfrac{3}{32}+\dfrac{3}{128}\)
4B - B = \(6-\dfrac{3}{512}\)
3B = \(\dfrac{3069}{512}\)
B= \(\dfrac{3069}{512}:3=\dfrac{1023}{512}\)