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a ,mẫu số chung nhỏ nhất là 35
b,mẫu số chung nhỏ nhất là 75
c,mẫu số chung nhỏ nhất là 24
a ,mẫu số chung nhỏ nhất là 35
b,mẫu số chung nhỏ nhất là 75
c,mẫu số chung nhỏ nhất là 24
a)
b) Mẫu số chung là BCNN (5, 6) = 30. Do đó:
c) Mẫu số chung là BCNN (7, 15) = 105. Do đó:
a) −55,87−55,87
Rút gọn: −55=−1−55=−1
MC: 7
Quy đồng ta được:
−1=−77−1=−77 và 8787
b) 3,−35,−563,−35,−56
MC: 30
Thừa số phụ thứ nhất là: 30: 1 = 30
Thừa số phụ thứ hai là: 30 : 5 = 6
Thừa số phụ thứ ba là: 30 : 6 = 5
Quy đồng ta được:
3=31=3.303=903−35=(−3).65.6=−1830−56=(−5).56.5=−25303=31=3.303=903−35=(−3).65.6=−1830−56=(−5).56.5=−2530
c) −97,−1915,−1−97,−1915,−1
MC: 15. 7 = 105
Thừa số phụ thứ nhất là: 105 : 7 = 15
Thừa số phụ thứ hai là: 105 : 15 = 7
Thừa số phụ thứ ba là: 105 : 1 = 105
Quy đồng ta được:
−97=(−9).157.15=−135105−1915=(−19).715.7=−133105−1=−11=(−1).1051.105=−105105−97=(−9).157.15=−135105−1915=(−19).715.7=−133105−1=−11=(−1).1051.105=−105105
a)
\(P=\dfrac{5}{6}+\dfrac{5}{12}+\dfrac{5}{20}+\dfrac{5}{30}+\dfrac{5}{42}+\dfrac{5}{56}+\dfrac{5}{72}+\dfrac{5}{90}\\ =\dfrac{5}{2.3}+\dfrac{5}{3.4}+\dfrac{5}{4.5}+\dfrac{5}{5.6}+\dfrac{5}{6.7}+\dfrac{5}{7.8}+\dfrac{5}{8.9}+\dfrac{5}{9.10}\\ \Rightarrow\dfrac{1}{5}P=\dfrac{1}{2.3}+\dfrac{1}{3.4}+\dfrac{1}{4.5}+...+\dfrac{1}{9.10}\\ =\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{9}-\dfrac{1}{10}\\ =\dfrac{1}{2}-\dfrac{1}{10}=\dfrac{5}{10}-\dfrac{1}{10}\\ =\dfrac{4}{10}=\dfrac{2}{5}\\ \Rightarrow P=\dfrac{2}{5}\cdot5=2\)
a) \(\dfrac{5}{3}+\dfrac{3}{-4}+\dfrac{7}{6}\) \(\left(MC:12\right)\)
\(=\dfrac{20}{12}+\dfrac{-9}{12}+\dfrac{14}{12}\)
\(=\dfrac{20+\left(-9\right)+14}{12}\)
\(=\dfrac{25}{12}\)
b) \(\dfrac{-1}{5}+\dfrac{5}{3}+\dfrac{-3}{2}\) \(\left(MC:30\right)\)
\(=\dfrac{-6}{30}+\dfrac{50}{30}+\dfrac{-45}{30}\)
\(=\dfrac{\left(-6\right)+50+\left(-45\right)}{30}\)
\(=\dfrac{-1}{30}\)
c) \(\dfrac{2}{7}+\dfrac{-7}{5}+\dfrac{-2}{35}\) \(\left(MC:35\right)\)
\(=\dfrac{10}{35}+\dfrac{-49}{35}+\dfrac{-2}{35}\)
\(=\dfrac{10+\left(-49\right)+\left(-2\right)}{35}\)
\(=\dfrac{-41}{35}\)
d) \(3+\dfrac{-7}{2}+\dfrac{-1}{5}\) \(\left(MC:10\right)\)
\(=\dfrac{30}{10}+\dfrac{-35}{10}+\dfrac{-2}{10}\)
\(=\dfrac{30+\left(-35\right)+\left(-2\right)}{10}\)
\(=\dfrac{-7}{10}\)
a) \(\dfrac{5}{3}+\dfrac{3}{-4}+\dfrac{7}{6}\)
\(=\dfrac{5}{3}+\dfrac{-3}{4}+\dfrac{7}{6}\)
\(=\) \(\dfrac{20}{12}+\dfrac{-9}{12}+\dfrac{14}{12}\)
\(=\dfrac{11}{12}+\dfrac{14}{12}\)
\(=\dfrac{25}{12}\)
b) \(\dfrac{-1}{5}+\dfrac{5}{3}+\dfrac{-3}{2}\)
\(=\dfrac{-6}{30}+\dfrac{50}{30}+\dfrac{-45}{30}\)
\(=\dfrac{44}{30}+\dfrac{-45}{30}\)
\(=\dfrac{-1}{30}\)
c) \(\dfrac{2}{7}+\dfrac{-7}{5}+\dfrac{-2}{35}\)
\(=\dfrac{10}{35}+\dfrac{-49}{35}+\dfrac{-2}{35}\)
\(=\dfrac{-39}{35}+\dfrac{-2}{35}\)
\(=\dfrac{-41}{35}\)
d) \(3+\dfrac{-7}{2}+\dfrac{-1}{5}\)
\(=\dfrac{3}{1}+\dfrac{-7}{2}+\dfrac{-1}{5}\)
\(=\dfrac{30}{10}+\dfrac{-35}{10}+\dfrac{-2}{10}\)
\(=\dfrac{-5}{10}+\dfrac{-2}{10}\)
\(=\dfrac{-7}{10}\)
a: 2/9=4/18
1/3=6/18
5/18=5/18
b: 7/15=14/30
1/5=6/30
-5/6=-25/30
c: -21/56=-3/7
-3/16=-63/336
5/24=70/336
-21/56=-3/7=-144/336
d: \(\dfrac{-4}{7}=\dfrac{-36}{63}\)
8/9=56/63
\(-\dfrac{10}{21}=-\dfrac{30}{63}\)
e: 3/-20=-3/20=-9/60
-11/-30=11/30=22/60
7/15=28/60
a) \(4,5:\left[\left(\dfrac{9-10}{6}\right)-\dfrac{9}{5}+\dfrac{12}{5}\right]-\dfrac{1}{7}\)
\(=4,5:\left(\dfrac{-1}{6}-\dfrac{-3}{5}\right)-\dfrac{1}{7}\)
=\(4,5:\left(\dfrac{-5+18}{30}\right)-\dfrac{1}{7}\)
=\(4,5:\dfrac{13}{30}-\dfrac{1}{7}\)=\(\dfrac{135}{13}-\dfrac{1}{7}=\dfrac{932}{91}\)
b) \(\dfrac{13}{3}:\left(\dfrac{1}{4}+\dfrac{5}{4}\right)-\dfrac{20}{3}\)
=\(\dfrac{13}{3}.\dfrac{2}{3}-\dfrac{20}{3}\)=\(\dfrac{26}{9}-\dfrac{20}{3}=\dfrac{26}{9}-\dfrac{60}{9}=\dfrac{-34}{9}\)
c) \(5.\left(\dfrac{1}{1.4}+\dfrac{1}{4.7}+.....+\dfrac{1}{91.94}\right)\)
\(=5.\left[\dfrac{1}{3}\left(1-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{7}+...+\dfrac{1}{91}-\dfrac{1}{94}\right)\right]\)
\(=5.\left[\dfrac{1}{3}.\left(1-\dfrac{1}{94}\right)\right]\)
=\(5.\left(\dfrac{1}{3}.\dfrac{93}{94}\right)\)
\(=5.\dfrac{31}{94}=\dfrac{155}{94}\)
Chúc bạn học tốt 
\(B=\dfrac{5}{70}+\dfrac{5}{130}+\dfrac{5}{208}+\dfrac{5}{304}+\dfrac{5}{418}\\ =\dfrac{5}{7.10}+\dfrac{5}{10.13}+\dfrac{5}{13.16}+\dfrac{5}{16.19}+\dfrac{5}{19.22}\\ =\dfrac{5}{3}\left(\dfrac{3}{7.10}+\dfrac{3}{10.13}+\dfrac{3}{13.16}+\dfrac{3}{16.19}+\dfrac{3}{19.22}\right)\\ =\dfrac{5}{3}\left(\dfrac{1}{7}-\dfrac{1}{10}+\dfrac{1}{10}-\dfrac{1}{13}+\dfrac{1}{13}-\dfrac{1}{16}+\dfrac{1}{16}-\dfrac{1}{19}+\dfrac{1}{19}-\dfrac{1}{22}\right)\\ =\dfrac{5}{3}\left(\dfrac{1}{7}-\dfrac{1}{22}\right)=\dfrac{5}{3}.\dfrac{15}{154}=\dfrac{25}{154}\)
\(B=\dfrac{5}{70}+\dfrac{5}{130}+\dfrac{5}{208}+\dfrac{5}{304}+\dfrac{5}{418}\)
\(B=\dfrac{5}{7.10}+\dfrac{5}{10.13}+\dfrac{5}{13.16}+\dfrac{5}{16.19}+\dfrac{5}{19.22}\)
\(B=\dfrac{5}{3}.\left(\dfrac{1}{7.10}+\dfrac{1}{10.13}+\dfrac{1}{13.16}+\dfrac{1}{16.19}+\dfrac{1}{19.22}\right)\)
\(B=\dfrac{5}{3}.\left(\dfrac{1}{7}-\dfrac{1}{10}+\dfrac{1}{10}-\dfrac{1}{13}+...+\dfrac{1}{19}-\dfrac{1}{22}\right)\)
\(B=\dfrac{5}{3}.\left(\dfrac{1}{7}-\dfrac{1}{22}\right)\)
\(B=\dfrac{5}{3}.\dfrac{15}{154}\)
\(B=\dfrac{25}{154}\)
$\color {#B0E0E6} {\text{@456}}$