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dấu . là dấu nhân đó mk viết nhầm 

\(A=18\cdot\left(\frac{373737}{424242}+\frac{373737}{565656}+\frac{373737}{727272}\right)\)

\(=18\cdot\left(\frac{37}{42}+\frac{37}{56}+\frac{37}{72}\right)=18\cdot37\cdot\left(\frac{1}{42}+\frac{1}{56}+\frac{1}{72}\right)\)

\(=18\cdot37\cdot\left(\frac{1}{6\cdot7}+\frac{1}{7\cdot8}+\frac{1}{8\cdot9}\right)\)

\(=18\cdot37\cdot\left(\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}\right)\)

\(=18\cdot37\cdot\left(\frac{1}{6}-\frac{1}{9}\right)=18\cdot37\cdot\left(\frac{3}{18}-\frac{2}{18}\right)\)

\(=18\cdot37\cdot\frac{1}{18}=18\cdot\frac{1}{18}\cdot37=1\cdot37=37\)

\(27\cdot\left(\dfrac{373737}{424242}+\dfrac{373737}{565656}+\dfrac{373737}{727272}\right)\\ =27\cdot\left(\dfrac{37}{42}+\dfrac{37}{56}+\dfrac{37}{72}\right)\\ =27\cdot37\cdot\left(\dfrac{1}{42}+\dfrac{1}{56}+\dfrac{1}{72}\right)\\ =999\cdot\left(\dfrac{1}{6\cdot7}+\dfrac{1}{7\cdot8}+\dfrac{1}{8\cdot9}\right)\\ =999\cdot\left(\dfrac{1}{6}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{9}\right)\\ =999\cdot\left(\dfrac{1}{6}-\dfrac{1}{9}\right)\\ =999\cdot\dfrac{1}{18}\\ =\dfrac{111}{2}\)

\(27.\left(\dfrac{373737}{424242}+\dfrac{373737}{565656}+\dfrac{373737}{727272}\right)=\dfrac{111}{2}\)

\(A=18x\left(\frac{373737}{424242}+\frac{373737}{565656}+\frac{373737}{727272}\right)\)

    \(=18x\left(\frac{37x10101}{42x10101}+\frac{37x10101}{56x10101}+\frac{37x10101}{72x10101}\right)\)

     \(=18x\left(\frac{37}{42}+\frac{37}{56}+\frac{37}{72}\right)\)

       \(=18x\frac{37}{18}\)

        \(=37\)

37/42+37/56+37/72 = 37(1/42+1/56+1/72)=37( 1/6.7 + 1/7.8 + 1/8.9 )= 37( 1/6 - 1/7 + 1/7 - 1/8 + 1/8 - 1/9)= 37( 1/6 - 1/9)= 37.1/18=37/18

B = 37/42 + 37/56 + 37/72 

B = 37/18

37/42 + 37/56 +37/72=37/24 + 37/72= 37/18

rut gon

= (57/42+57/56+57/72)*18

=19/6*18

=57