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Bài 3:
a: \(\dfrac{3}{5}-x=\dfrac{-1}{4}+\dfrac{7}{10}=\dfrac{-5}{20}+\dfrac{14}{20}=\dfrac{9}{20}\)
=>x=3/5-9/20=12/20-9/20=3/20
b: \(\dfrac{-5}{-8}-x=\dfrac{-5}{-6}+\dfrac{1}{8}\)
=>5/8-x=5/6+1/8
=>5/8-x=23/24
=>x=-1/3
c: \(8.25-x=3+\dfrac{1}{6}=\dfrac{19}{6}\)
=>x=33/4-19/6=99/12-38/12=61/12
A = 1/10 + 1/15 + 1/21 + ....+ 1/120
= 2/20 + 2/30 + 2/42 +....+ 2/240
= 2/4x5 + 2/5x6 + 2/6x7 + ...+ 2/15x16
= 2 x ( 1/4x5 + 1/5x6 + 1/6x7 +...+ 1/15x16)
=2 x ( 1/4 - 1/5 + 1/5 - 1/6 +1/6 - 1/7 + .....+ 1/15 - 1/16 )
= 2 x ( 1/4 - 1/16 )
= 2 x ( 4/16 - 1/16 )
= 2 x 3/16
= 6/16
= 3/8
tk nhé
C= -1 - 2 - 3 - 4 - ..... - 100
C = - (1 + 2 + 3 + ...... + 100)
C = - (100 x 101 : 2)
C= - (10100 : 2)
C = -5050
1) \(\frac{5}{8}.\frac{7}{3}-\frac{5}{2}.\frac{1}{8}=\frac{5}{8}.\frac{7}{3}-\frac{5}{8}.\frac{1}{2}=\frac{5}{8}\left(\frac{7}{3}-\frac{1}{2}\right)=\frac{5}{8}.\frac{11}{6}=\frac{55}{48}\)
2) \(\frac{21}{10}.\frac{3}{4}-\frac{21}{10}.\frac{3}{4}=\frac{21}{10}\left(\frac{3}{4}-\frac{3}{4}\right)=\frac{21}{10}.0=0\)
3) \(\frac{-4}{11}:\frac{-6}{11}=\frac{-4}{11}.\frac{-11}{6}=\frac{-4.\left(-11\right)}{11.6}=\frac{-4.\left(-1\right)}{1.6}=\frac{4}{6}=\frac{2}{3}\)
4)\(\frac{2}{7}.\frac{14}{3}-1=\frac{2.14}{7.3}-1=\frac{2.2}{1.3}-1=\frac{4}{3}-1=\frac{1}{3}\)
5)\(\frac{4}{7}:\left(\frac{1}{5}.\frac{4}{7}\right)=\frac{4}{7}:\frac{4}{35}=\frac{4}{7}.\frac{35}{4}=\frac{4.35}{7.4}=\frac{1.5}{1.1}=5\)
6) \(\frac{12}{7}.\frac{7}{4}+\frac{35}{11}:\frac{245}{121}=\frac{12.7}{7.4}+\frac{35}{11}.\frac{121}{245}=\frac{3.1}{1.1}+\frac{35}{11}.\frac{121}{245}=3+\frac{35}{11}.\frac{121}{245}=3+\frac{35.121}{11.245}=\frac{1.11}{1.7}=\frac{11}{7}\)