\(\dfrac{7}{30}=\dfrac{21}{90};\dfrac{4}{45}=\dfrac{8}{90}\)

Bài làm:

Rút gọn: \(\frac{-21}{-140}=\frac{3}{20};\frac{5\cdot2-5,3\cdot2}{5\cdot3+5\cdot2\cdot3\cdot2}=-\frac{1}{125}\)

Phân tích thừa số nguyên tố: \(20=2^2\cdot5;75=3\cdot5^2;125=5^3\)

=> MSC (20; 75; 125) = 2². 3. 5³ = 1500

Quy đồng:

\(\frac{-21}{-140}=\frac{3}{20}=\frac{225}{1500}\)

\(\frac{13}{-75}=\frac{-13}{75}=-\frac{260}{1500}\)

\(\frac{5\cdot2-5,3\cdot2}{5\cdot3+5\cdot2\cdot3\cdot2}=-\frac{1}{125}=-\frac{12}{1500}\)

\(a,\dfrac{52}{32}=\dfrac{13}{8}\\ \dfrac{55}{65}=\dfrac{11}{13}\\ MSC:104\\ =>\dfrac{13}{8}=\dfrac{13.13}{8.13}=\dfrac{169}{104}\\ \dfrac{11}{13}=\dfrac{11.8}{13.8}=\dfrac{88}{104}\)

\(b,\dfrac{72}{64}=\dfrac{9}{8}\\ \dfrac{54}{81}=\dfrac{2}{3}\\ MSC:24\\ \dfrac{9}{8}=\dfrac{9.3}{8.3}=\dfrac{27}{24}\\ \dfrac{2}{3}=\dfrac{2.8}{3.8}=\dfrac{16}{24}\\ c,\dfrac{24}{36}=\dfrac{2}{3}\\ \dfrac{12}{30}=\dfrac{2}{5}\\ MSC:15\\ \dfrac{2}{3}=\dfrac{2.5}{3.5}=\dfrac{10}{15}\\ \dfrac{2}{5}=\dfrac{2.3}{3.5}=\dfrac{6}{15}\)

\(d,\dfrac{80}{40}=2\\ \dfrac{45}{63}=\dfrac{5}{7}\\ MSC:7\\ 2=\dfrac{14}{7}\\ e,\dfrac{150}{50}=3\\ \dfrac{100}{200}=\dfrac{1}{2}\\ MSC:2\\ 3=\dfrac{6}{2}\\ f,\dfrac{54}{81}=\dfrac{2}{3}\\ \dfrac{69}{93}=\dfrac{23}{21}\\ MSC:21\\ \dfrac{2}{3}=\dfrac{2.7}{3.7}=\dfrac{14}{21}\)

\(a.\dfrac{52}{32}\text{=}\dfrac{13}{8};\dfrac{55}{65}\text{=}\dfrac{11}{13}\)

\(\dfrac{13.13}{13.8}\text{=}\dfrac{169}{104};\dfrac{11.8}{13.8}\text{=}\dfrac{88}{104}\)

\(c.\dfrac{24}{36}\text{=}\dfrac{2}{3};\dfrac{12}{30}\text{=}\dfrac{2}{5}\)

\(\dfrac{2.5}{3.5}\text{=}\dfrac{10}{15};\dfrac{2.3}{5.3}\text{=}\dfrac{6}{15}\)

\(b.\dfrac{72}{64}\text{=}\dfrac{9.3}{8.3}\text{=}\dfrac{27}{24};\dfrac{54}{81}\text{=}\dfrac{2.8}{3.8}\text{=}\dfrac{16}{24}\)

\(d.\dfrac{80}{40}\text{=}\dfrac{2.63}{63}\text{=}\dfrac{126}{63};\dfrac{45}{63}\)

\(e.\dfrac{150}{50}\text{=}\dfrac{3.2}{2}\text{=}\dfrac{6}{2};\dfrac{100}{200}\text{=}\dfrac{1}{2}\)

\(f.\dfrac{54}{81}\text{=}\dfrac{2.31}{3.31}\text{=}\dfrac{62}{93};\dfrac{69}{93}\)

Đáp án cần chọn là: A

3   .   4 − 3   .   7 6   .   5 + 9 = 12 − 21 30 + 9 = − 9 39 = − 3 13 3   .   4 − 3   .   7 6   .   5 + 9 = 12 − 21 30 + 9 = − 9 39 = − 3 13 6   .   9 − 2   .   17 63   .   3 − 119 = 54 − 34 189 − 119 = 20 70 = 2 7

MSC = 91

− 3 13 = − 3.7 13.7 = − 21 91 ;     2 7 = 2.13 7.13 = 26 91

Vậy sau khi quy đồng ta được hai phân số  − 21 91  và  26 91

a. 10/13 + 9/16         
=160/208+117/208
=277/208
7/90 + 123/270
=7/90+41/90
=48/90
=8/15

b. 19/21 - 5/12                 
=76/84-35/84
=41/84
 90/100 - 25/40
=9/10-25/40
=36/40-25/40
=11/40

a. 10/13 + 9/16         
=160/208+117/208
=277/208
7/90 + 123/270
=7/90+41/90
=48/90
=8/15

b. 19/21 - 5/12                 
=76/84-35/84
=41/84
 90/100 - 25/40
=9/10-25/40
=36/40-25/40
=11/40

a) \(\dfrac{10}{13}+\dfrac{9}{16}\)                         \(\dfrac{7}{90}+\dfrac{123}{270}\)

= \(\dfrac{160}{208}+\dfrac{117}{208}\)                   = \(\dfrac{7}{90}+\dfrac{41}{90}\)

= \(\dfrac{277}{208}\)                              = \(\dfrac{8}{15}\)

b) \(\dfrac{19}{21}-\dfrac{5}{12}\)                       \(\dfrac{90}{100}-\dfrac{25}{40}\)

= \(\dfrac{76}{84}-\dfrac{35}{84}\)                      = \(\dfrac{9}{10}-\dfrac{25}{40}\)

= \(\dfrac{41}{84}\)                               = \(\dfrac{36}{40}-\dfrac{25}{40}\)

                                      = \(\dfrac{11}{40}\)

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