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

2/5+1

2/5+1/1

2/5+5/5

7/5

60/8-6/8-15/20

27/4-15/20

135/20-15/20

6

\(\frac{2}{5}+\frac{3}{4}+\frac{1}{4}\)

\(=\frac{2}{5}+1\)

\(=\frac{2}{5}+\frac{5}{5}\)

\(=\frac{7}{5}\)

\(\frac{30}{4}-\frac{6}{8}-\frac{15}{20}\)

\(=\frac{30}{4}-\frac{3}{4}-\frac{3}{4}\)

\(=\frac{24}{4}=6\)

Chúc bạn học tốt

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

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

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

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

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

\(=4\)

=A x (16-9-7): (2+4+6+8+...+20)

=(A x 0): (2+4+6+8+...+20)

=0 x (2+4+6+8+...+20)

=0

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 : \(1+2+3+...+19+20\)

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

= (5/6 + 7/6) + (4/15 + 11/15)
= 2 + 1

= 3 

=(5/6+7/6)+(4/15+11/15)

=12/6+15/15

=2+1

=3

đúng ko day

\(=\left(\dfrac{5}{6}+\dfrac{7}{6}\right)+\left(\dfrac{4}{15}+\dfrac{11}{15}\right)=\dfrac{12}{6}+1=2+1=3\)

=(5/6+7/6)+(4/15+11/15)

=12/6+15/15

=2+1

=3

= (5/6 + 7/6) + (11/15 + 4/15)

= 12/6 + 15/15

= 2 + 1

= 3