36-4x9 = 36 - 36 = 0 Vì số nào nhân với 0 thì cũng bằng 0 nên phép tính đó bằng 0

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

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

bạn tính cẩn thận thì sẽ ra bài này rất dễ

\(\frac{5}{15}+\frac{3}{15}+\frac{4}{20}+\frac{18}{27}+\frac{12}{16}+\frac{20}{25}\\ =\frac{1}{3}+\frac{1}{5}+\frac{1}{5}+\frac{2}{3}+\frac{3}{4}+\frac{4}{5}\\ \)

\(=\left(\frac{1}{3}+\frac{2}{3}\right)+\left(\frac{1}{5}+\frac{1}{5}+\frac{4}{5}\right)+\frac{3}{4}\\ =1+\frac{5}{6}+\frac{3}{4}=2\frac{7}{12}=\frac{31}{12}\)

#Châu's ngốc

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 : \(1+2+3+...+19+20\)

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

Bài 1:
\(\dfrac{36}{21}-\dfrac{15}{20}\)
\(=\dfrac{12}{7}-\dfrac{3}{4}\)
\(=\dfrac{48}{28}-\dfrac{21}{28}\)
\(=\dfrac{27}{28}\)
\(----\)
\(\dfrac{63}{45}-\dfrac{20}{25}\)
\(=\dfrac{63}{45}-\dfrac{4}{5}\)
\(=\dfrac{63}{45}-\dfrac{36}{45}\)
\(=\dfrac{27}{45}=\dfrac{3}{5}\)
Bài 2:
\(\dfrac{3}{5}+\dfrac{3}{16}+\dfrac{13}{16}\)
\(=\dfrac{3}{5}+\left(\dfrac{3}{16}+\dfrac{13}{16}\right)\)
\(=\dfrac{3}{5}+1\)
\(=\dfrac{3}{5}+\dfrac{5}{5}\)
\(=\dfrac{8}{5}\)

`36/21-15/20`

`=12/7-3/4`

`=48/28-21/28`

`=27/28`

__

`63/45-20/25`

`=7/5-4/5`

`=3/5`

__

`2)`

`3/5+3/16+13/16`

`=3/5+(3/16+13/16)`

`=3/5+16/16`

`=3/5+1`

`=3/5+5/5`

`=8/5`

`#QiN`

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

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

\(=4\)

(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

các bạn nhanh nhanh nha

các bạn trả lời câu hỏi trước 8 giờ

SORRY QUÊN CÁCH LÀM RỒI