a) 2/3 : 3/5 × 5/7 : 2/3

= 2/3 × 5/3 × 5/7 × 3/2 

= 25/21

b) 1 1/2 × 1 1/3 × 1 1/18 × 1 1/15 × 1 1/24 × 1 1/35

= 3/2 × 4/3 × 19/18 × 16/15 × 25/24 × 36/35

= 2 × 152/35 × 15/14

= 304/35 × 15/14

= 152/7

\(=\dfrac{4}{3}\cdot\dfrac{9}{8}\cdot\dfrac{16}{15}\cdot\dfrac{25}{24}\cdot\dfrac{36}{35}=\dfrac{12}{7}\)

= 4/3*9/8*16/15*25/24*36/35

=2*2/1*3 * 3*3/2*4 *4*4/3*5 *5*5/4*6 * 6*6/5*7

= (2*3*4*5*6 / 1*2*3*4*5) * ( 2*3*4*5*6 / 3*4*5*6*7)

=6/1* 2/7 

= 12/7

ban lam sai rui de mk lam lai nhe.

\(12.\left(x-1\right):3=4^3-2^3\)

\(12.\left(x-1\right):3=64-8\)

\(12.\left(x-1\right):3=56\)

\(12.\left(x-1\right)=56.3\)

\(12.\left(x-1\right)=168\)

\(x-1=168:12\)

\(x-1=14\)

             \(x=15\)

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1/3x1/8x...x1/99

=1/(1x3)x1/(2x4)x...x1/(9x11)

=1/(1x3x2x4x...x9x11)

=1/(1x2x3x3x4x4x5x5x...x9x9x10x11)