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mk lm k nổi đâu
Dài quá nhìn lòi bảng họng lun ak
Bài : 4
a/ \(\frac{1}{5\cdot6}+\frac{1}{6\cdot7}+\frac{1}{7\cdot8}+....+\frac{1}{24\cdot25}\)
\(=\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+....+\frac{1}{24}-\frac{1}{25}\)
\(=\frac{1}{5}-\frac{1}{25}\)
\(=\frac{4}{25}\)
b/ \(\frac{2}{1\cdot3}+\frac{2}{3\cdot5}+\frac{2}{5\cdot7}+....+\frac{2}{99\cdot101}\)
\(=\frac{3-1}{1\cdot3}+\frac{5-3}{3\cdot5}+\frac{7-5}{5\cdot7}+...+\frac{101-99}{99\cdot101}\)
\(=\frac{1}{1}-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+....+\frac{1}{99}-\frac{1}{101}\)
\(=\frac{1}{1}-\frac{1}{101}\)
\(=\frac{100}{101}\)
c/ \(\frac{5^2}{1\cdot6}+\frac{5^2}{6\cdot11}+\frac{5^2}{11\cdot16}+\frac{5^2}{16\cdot21}+\frac{5^2}{21\cdot26}+\frac{5^2}{26\cdot31}\)
\(=\frac{25}{1\cdot6}+\frac{25}{6\cdot11}+\frac{25}{11\cdot16}+\frac{25}{16\cdot21}+\frac{25}{21\cdot26}+\frac{25}{26\cdot31}\)
\(=\frac{6-1}{1\cdot6}+\frac{11-6}{6\cdot11}+....+\frac{31-26}{26\cdot31}\)
\(=\frac{25}{5}\cdot\left(\frac{1}{1}-\frac{1}{6}+\frac{1}{6}-\frac{1}{11}+....+\frac{1}{26}-\frac{1}{31}\right)\)
\(=\frac{25}{5}\cdot\left(\frac{1}{1}-\frac{1}{31}\right)\)
\(=\frac{25}{5}\cdot\frac{30}{31}\)
\(=\frac{150}{31}\)
d/ \(\frac{3}{1\cdot3}+\frac{3}{3\cdot5}+\frac{3}{5\cdot7}+....+\frac{3}{49\cdot51}\)
\(=\frac{3-1}{1\cdot3}+\frac{5-3}{3\cdot5}+\frac{7-5}{5\cdot7}+....+\frac{51-49}{49\cdot51}\)
\(=\frac{3}{2}\cdot\left(\frac{1}{1}-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+....+\frac{1}{49}-\frac{1}{51}\right)\)
\(=\frac{3}{2}\cdot\left(\frac{1}{1}-\frac{1}{51}\right)\)
\(=\frac{3}{2}\cdot\frac{50}{51}\)
\(=\frac{25}{17}\)
e/ \(\frac{1}{7}+\frac{1}{91}+\frac{1}{247}+\frac{1}{475}+\frac{1}{775}+\frac{1}{1147}\)
\(=\frac{1}{1\cdot7}+\frac{1}{7\cdot13}+\frac{1}{13\cdot19}+\frac{1}{19\cdot25}+\frac{1}{25\cdot31}+\frac{1}{31\cdot37}\)
\(=\frac{7-1}{1\cdot7}+\frac{13-7}{7\cdot13}+....+\frac{37-31}{31\cdot37}\)
\(=\frac{1}{6}\cdot\left(1-\frac{1}{7}+\frac{1}{7}-\frac{1}{13}+....+\frac{1}{31}-\frac{1}{37}\right)\)
\(=\frac{1}{6}\cdot\left(1-\frac{1}{37}\right)\)
\(=\frac{1}{6}\cdot\frac{36}{37}\)
\(=\frac{6}{37}\)
a) ta có : 3/4 = -x/4
=> -x = 3×4/4
=> -x =3
=> x = -3
Mặt khác: -x/4 =21/y
Với x = -3, ta có :
-3/4 = 21/y
=> y = 21×4/-3 = -28
Lại có : 21/y = z/-80
Với y = -28, ta có:
22/-28 = z/-80
=> z = 21×-80/-28 = 60
Vậy x= -3; y = -28; z = 60
b) Ta có: y-2/2 = 18/-2
=> y -2 = 2×18/-2
=> y-2 = -18 => y = -16
Lại có : x/3 = y-2/2
Với y = -16, ta có:
x/3 = -16-2/2
=> x/3 = -18/2
=> x = 3×-18/2 => x = -27
Vậy x = -27; y = -16
Mk cx giống pn Cao Hoàng Minh Nguyệt z đó!!! Mk qên lun ùi!!!
a.\(\frac{2}{9}+\frac{5}{12}+\frac{3}{4}=\frac{8}{36}+\frac{15}{36}+\frac{27}{36}=\frac{25}{18}\)
b. \(\frac{3}{5}+\frac{7}{10}+\frac{13}{20}=\frac{12}{20}+\frac{14}{20}+\frac{13}{20}=\frac{39}{20}\)
c.\(\frac{3}{4}-\frac{1}{3}-\frac{5}{18}=\frac{29}{36}-\frac{12}{36}-\frac{10}{36}=\frac{7}{36}\)
d.\(\frac{3}{14}+\frac{5}{8}-\frac{1}{2}=\frac{24}{112}+\frac{70}{112}-\frac{56}{112}=\frac{19}{56}\)
e.\(\frac{1}{2}-\frac{1}{3}+\frac{1}{4}+\frac{1}{6}=\frac{6}{12}-\frac{4}{12}+\frac{3}{12}+\frac{2}{12}=\frac{7}{12}\)
là 12
nha bn
Học tốt
MSC nhỏ nhất là 12
Cách giải:
PT 3; 2; 4 ra TSNT ta đc
\(3=1.3\)
\(2=1.2\)
\(4=2^2\)
\(BCNN\left(2;3;4\right)=1.2^2.3=12\)