\(\dfrac{15}{8}\) : \(\dfrac{5}{20}\) - \(\dfrac{7}{8}\) : \(\dfrac{1}{4}\) 

= \(\dfrac{15}{8}\) : \(\dfrac{1}{4}\) - \(\dfrac{7}{8}\) : \(\dfrac{1}{4}\)

= ( \(\dfrac{15}{8}\) - \(\dfrac{7}{8}\)) : \(\dfrac{1}{4}\)

= \(\dfrac{8}{8}\) \(\times\) 4

= 4

1/3+1/4+1/5+8/10+15/20+20/30

= 1/3+1/4+1/5+4/5+3/4+2/3

= (1/3+2/3)+(1/4+3/4)+(1/5+4/5)

= 1 + 1 + 1

=3

a) \(\dfrac{3}{5}\times\dfrac{17}{21}+\dfrac{17}{21}\times\dfrac{2}{5}\)

\(=\dfrac{17}{21}\times\left(\dfrac{3}{5}+\dfrac{2}{5}\right)\)

\(=\dfrac{17}{21}\times1\)

\(=\dfrac{17}{21}\)

b) \(\dfrac{15}{8}:\dfrac{5}{20}-\dfrac{7}{8}:\dfrac{1}{4}\)

\(=\dfrac{15}{8}\times\dfrac{20}{5}-\dfrac{7}{8}\times4\)

\(=\dfrac{15}{8}\times4-\dfrac{7}{8}\times4\)

\(=4\times\left(\dfrac{15}{8}-\dfrac{7}{8}\right)\)

\(=4\times1\)

\(=4\)

a, =(3/5+2/5)x17/21

=1x17/21

=17/21

a) \(\dfrac{3}{5}\) x \(\dfrac{17}{21}+\dfrac{17}{21}\) x \(\dfrac{2}{5}\)

\(=\left(\dfrac{3}{5}+\dfrac{2}{5}\right)\) x \(\dfrac{17}{21}\)

\(=\dfrac{5}{5}\) x \(\dfrac{17}{21}\)

\(=1\) x \(\dfrac{17}{21}\)

\(=\dfrac{17}{21}\)

b) \(\dfrac{15}{8}:\dfrac{5}{20}-\dfrac{7}{8}:\dfrac{1}{4}\)

\(=\dfrac{15}{8}:\dfrac{1}{4}-\dfrac{7}{8}:\dfrac{1}{4}\)

\(=\left(\dfrac{15}{8}-\dfrac{7}{8}\right):\dfrac{1}{4}\)

\(=\dfrac{8}{8}:\dfrac{1}{4}\)

\(=1:\dfrac{1}{4}\)

\(=\dfrac{1}{1}:\dfrac{1}{4}\)

\(=\dfrac{1}{1}\) x \(\dfrac{4}{1}\)

\(=\dfrac{4}{1}\)

\(=4\)

39200

Ghi đầy đủ bài giải 

1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20

=(1+19)+(2+18)+(3+17)+(4+16)+(5+15)+(6+14)+(7+13)+(8+12)+(9+11)+20+10

=20+20+20+20+20+20+20+20+20+20+10

=20×10+10

=200+10

=210

= ( 1 + 19 ) + ( 2 + 18 ) + ( 3 + 17 ) + (4 + 16 ) + ( 5 + 15 ) + ( 6 + 14 ) + ( 7 + 13 ) + ( 8 + 12) + ( 9+ 11 ) + 20

= 20 + 20 + 20 +20 +  20 +20 + 20 + 20 + 20 +20

= 20 x 10

= 200

tui đầu tiên đó 

Ta có : \(1+2+3+...+19+20\)

          \(=\frac{\left(20+1\right).20}{2}=\frac{21.20}{2}=210\)

8/15 + 7/4 + 7/15 + 5/4

= 8/15 + 7/15 + 7/4 +5/4

= ( 8/15 + 7/15) + ( 7/4 + 5/4)

= 15/15 + 12/4

= 1 + 3

=4

=8/15+7/15+7/4+5/4

=1+3

=4

(30/4-15/20)-6/8=27/4-6/8=6

30/4 - 6/8 - 15/20 = 30/4 - (6/8 - 15/20) = 30/4 - (3/4 - 3/4) = 30/4 - 0= 30/4

\(\dfrac{30}{4}-\dfrac{6}{8}-\dfrac{15}{20}=\dfrac{30}{4}-\dfrac{3}{4}-\dfrac{3}{4}=6\)

a, 6/7*8/15*7/6*15/16

=(6/7*7/6)*(8/15*15/16)

=1*1/2

=1/2

b, Đề bài hơi sai

câu b mình sửa lại là 19/20 x 13/21 + 19/20 x 8/21

b, 19/20*13/21+19/20*8/21

=19/20*(13/21+8/21)

=19/20*21/21

=19/20*1

=19/20

TK nha

(19+1)+(18+2)+(17+3)+(16+4)+(15+5)+(14+6)+(13+7)+(12+8)+(11+9)+20)+10

=20+20+20+20+20+20+20+20+20+20=(20*10)+10

=210

1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20

=(1+19)+(2+18)+(3+17)+(4+16)+(5+15)+(6+14)+(7+13)+(8+12)+(9+11)+(10+20)

=20+20+20+20+20+20+20+20+20+30

=(20 x 9) + 30

=180 + 30

=210

tk nha

1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20

= (1+19)+(2+18)+(3+17)+(4+16)+(5+15)+(6+14)+(7+13)+(8+12)+(9+11)+20+10

= 20     +   20   +  20     +  20    +  20      +  20    +  20     +  20    +  20     +  20 + 10

= 10 . 10 +10 

=  100     +  10 = 110

Bài làm

   \(\frac{7}{15}x\frac{3}{8}+\frac{7}{15}x\frac{5}{8}\)

= \(\frac{7}{15}x\left(\frac{3}{8}+\frac{5}{8}\right)\)

= \(\frac{7}{15}x\frac{8}{8}\)

= \(\frac{7}{15}x1\)

= \(\frac{7}{15}\)

# Chúc bạn học tốt #

\(\frac{7}{15}\cdot\frac{3}{8}+\frac{7}{15}\cdot\frac{5}{8}\)

\(=\frac{7}{15}\cdot(\frac{3}{8}+\frac{5}{8})\)

\(=\frac{7}{15}\cdot1=\frac{7}{15}\)

\(\frac{7}{15}\times\frac{3}{8}+\frac{7}{15}\times\frac{5}{8}\)

\(=\frac{7}{15}\times\left(\frac{3}{8}+\frac{5}{8}\right)\)

\(=\frac{7}{15}\times1\)

\(=\).....................

Tự làm tiếp nhé!