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1.\(\dfrac{11}{18};\dfrac{7}{9};\dfrac{14}{15}\)
2.\(\dfrac{4}{9}+\dfrac{3}{7}=\dfrac{28}{63}+\dfrac{27}{63}=\dfrac{55}{63}\)
\(\dfrac{5}{6}-\dfrac{3}{8}=\dfrac{20}{24}-\dfrac{9}{24}=\dfrac{11}{24}\)
\(\dfrac{18}{25}\cdot\dfrac{15}{6}=\dfrac{18\cdot15}{25\cdot6}=\dfrac{270}{150}=\dfrac{9}{5}\)
\(\dfrac{30}{49}:\dfrac{6}{7}=\dfrac{30}{49}\cdot\dfrac{7}{6}=\dfrac{30\cdot7}{49\cdot6}=\dfrac{210}{294}=\dfrac{5}{7}\)
2:
a: =28/63+27/63=55/63
b: =20/24-9/24=11/24
c: =270/150=9/5
d: \(=\dfrac{30}{49}\cdot\dfrac{7}{6}=\dfrac{1}{7}\cdot5=\dfrac{5}{7}\)
2:
a/5-1/2=1/10
=>a/5=1/10+1/2=6/10=3/5
=>a=3
1:
4/9+1=4/9+9/9=13/9
3-1/2=6/2-1/2=5/2
2-3/4=8/4-3/4=5/4
2/5+1=2/5+5/5=7/5
b)1/2*2/3*3/4*4/5*5/6*6/7*7/8*8/9*9/10
=1x2x3x4x5x6x7x8x9
2x3x4x5x6x7x8x9x10
= 1
10
1+2+3+4+5+6+7+8+9+1+2+3+4+5+6+7+8+9+1+2+3+4+5+6+7+8+9=135
a) \(\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}\)
\(=1-\frac{1}{4}\)
\(=\frac{3}{4}\)
b) \(\frac{4}{7}\times\frac{8}{9}+\frac{4}{7}\times\frac{1}{9}\)
\(=\frac{4}{7}\times\left(\frac{8}{9}+\frac{1}{9}\right)\)
\(=\frac{4}{7}\times1\)
\(=\frac{4}{7}\)
c) \(\frac{13}{6}\times\frac{3}{8}-\frac{3}{8}\times\frac{7}{6}\)
\(=\frac{3}{8}\times\left(\frac{13}{6}-\frac{7}{6}\right)\)
\(=\frac{3}{8}\times1\)
\(=\frac{3}{8}\)
d) \(\frac{1}{1x2}+\frac{1}{2x3}+\frac{1}{3x4}+...+\frac{1}{9x10}\)
\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{9}-\frac{1}{10}\)
\(=1-\frac{1}{10}\)
\(=\frac{9}{10}\)
Lưu ý ^ bằng dấu nhân
\(\frac{4}{7}-\left(2×a+\frac{1}{6}\right):\frac{2}{3}=\frac{1}{8}\)
\(\left(2×a+\frac{1}{6}\right):\frac{2}{3}=\frac{4}{7}-\frac{1}{8}\)
\(\left(2×a+\frac{1}{6}\right):\frac{2}{3}=\frac{25}{56}\)
\(\left(2×a+\frac{1}{6}\right)=\frac{25}{56}×\frac{2}{3}\)
\(\left(2×a+\frac{1}{6}\right)=\frac{25}{84}\)
\(2×a=\frac{25}{84}-\frac{1}{6}\)
\(2×a=\frac{11}{84}\)
\(a=\frac{11}{84}:2\)
\(a=\frac{11}{168}\)
vậy \(a=\frac{11}{168}\)