a.$\frac{1}{21}$+$\frac{2}{21}$ +$\frac{3}{21}$ +...........+$\frac{18}{21}$ +$\frac{19}{21}$ +$\frac{20}{21}$
=$\frac{1+2+3+4+5+6+7+8.....+19+20}{21}$
=$\frac{( 19 + 1 ) + ( 18 + 2 ) + ( 17 + 3 ) + ... ( 11 + 9 ) + 20 + 10}{21}$
=$\frac{20 + 20 + 20 + ... + 20 + 20 + 10}{21}$
=$\frac{210}{21}$
=10
b.$\frac{1}{10}$ +$\frac{20}{100}$ +$\frac{300}{1000}$ $\frac{4000}{10000}$
=$\frac{1}{10}$ +$\frac{2}{10}$ +$\frac{3}{10}$ +$\frac{4}{10}$
=$\frac{1+2+3+4}{10}$
=$\frac{10}{10}$
=1
c.$\frac{4}{3 x 7}$ +$\frac{4}{7x11}$ +$\frac{4}{11x5}$ +$\frac{4}{15x19}$ +$\frac{4}{19x23}$ $\frac{4}{23x27}$
=$\frac{1}{3}$ -$\frac{1}{7}$ +$\frac{1}{7}$ -$\frac{1}{11}$ +$\frac{1}{11}$ -$\frac{1}{15}$+ $\frac{1}{15}$ -$\frac{1}{19}$ +$\frac{1}{19}$ - $\frac{1}{23}$ +$\frac{1}{23}$ -$\frac{1}{27}$
=$\frac{1}{3}$ - ($\frac{1}{7}$ +$\frac{1}{7}$ )-($\frac{1}{11}$ +$\frac{1}{11}$ )-($\frac{1}{15}$ - $\frac{1}{15}$ )-($\frac{1}{19}$ -$\frac{1}{19}$ )-$\frac{1}{23}$ -$\frac{1}{23}$)- $\frac{1}{27}$
=$\frac{1}{3}$ -$\frac{1}{27}$
=$\frac{8}{27}$
Bài 2:
a.
4823 + 1560 + 5177 + 8440
= ( 4823 + 5177 ) + ( 1560 + 8440 )
= 10000 + 10000
= 10000 x 2
= 20000
b. 10556 + 8074 + 9444 + 926 + 1000
= ( 10556 + 9444 ) + ( 8074 + 926) + 1000
= 20000 + 9000 +1000
= 20000 + 10000
= 30000
c.576+789+467+111
= ( 789 + 111 ) + 567 + 467
= 900 + 567 + 467
= 1467 + 467
= 1934
Bài 3
a.
21,251+6,058+0,749+1,042
=(21,251+0,749)+(6,058+1,042)
=22+7,1
=29,1
b.
1,53+5,039+12,47+5,691
=(1,53+12,47)+(5,039+5,691)
=13+10,73
=23,73
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