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ta có
a=1+2+4+...........+211
=1+[1+1]+[1+1+2]+[1+1+2+3]+..................+[1+1+2+3+...........+20] =[1+......+1]+[1+1+.....+1]+[2+......+2]+..........+[19+19]+20
=21+[1*20+2*19+...............+19*2+20]
=1561
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X:8x32+X:3x18-X:5x10=120
32X:8+18X:3-10X:5=120
4X+6X-2X =120
(4+6-2)X =120
8X =120
X =120:8
X =15
Chúc bn học tốt
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![](https://rs.olm.vn/images/avt/0.png?1311)
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\(\dfrac{8}{9}:\dfrac{3}{7}=\dfrac{56}{27}\\ \dfrac{8}{9}+\dfrac{2}{5}=\dfrac{58}{45}\\ \dfrac{7}{8}-\dfrac{1}{3}=\dfrac{13}{24}\\ \dfrac{3}{10}\times\dfrac{1}{6}=\dfrac{1}{20}\\ 1\dfrac{2}{7}+6\dfrac{5}{6}=\dfrac{9}{7}+\dfrac{41}{6}=\dfrac{341}{42}\\ 5\dfrac{3}{4}-\dfrac{1}{5}=\dfrac{23}{4}-\dfrac{1}{5}=\dfrac{111}{20}\\ 6\dfrac{2}{9}:4\dfrac{7}{10}=\dfrac{56}{9}:\dfrac{47}{10}=\dfrac{560}{423}\\ \dfrac{5}{3}+\dfrac{3}{2}-\dfrac{7}{6}=2\)
\(\dfrac{7}{3}\) x \(\dfrac{18}{5}\) = \(\dfrac{126}{15}\) = \(\dfrac{42}{5}\)
\(\dfrac{7}{3}\times\dfrac{18}{5}=\dfrac{7\times6\times3}{3\times5}=\dfrac{42}{5}\)