a; \(\dfrac{1}{4}\) + \(\dfrac{2}{5}\) + \(\dfrac{6}{8}\) + \(\dfrac{9}{15}\) + \(\dfrac{8}{1}\)

= (\(\dfrac{1}{4}\) + \(\dfrac{6}{8}\)) + (\(\dfrac{2}{5}\) + \(\dfrac{9}{15}\)) + \(\dfrac{8}{1}\)

= (\(\dfrac{1}{4}\) + \(\dfrac{3}{4}\)) + (\(\dfrac{2}{5}\) + \(\dfrac{3}{5}\)) + 8

=  1 + 1 + 8

=  2 + 8

= 10

b; \(\dfrac{1}{2}\) + \(\dfrac{2}{4}\) + \(\dfrac{3}{6}\) + \(\dfrac{4}{8}\) + \(\dfrac{5}{10}\) + \(\dfrac{6}{12}\) + \(\dfrac{7}{14}\) + \(\dfrac{8}{16}\) + \(\dfrac{10}{20}\)

=  \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) x (\(\dfrac{2}{2}\) + \(\dfrac{3}{3}\) + \(\dfrac{4}{4}\) + \(\dfrac{5}{5}\)+ \(\dfrac{6}{6}+\dfrac{7}{7}+\dfrac{8}{8}\) + \(\dfrac{10}{10}\))

= \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) x (1 + 1 +1 + 1+ 1+ 1+ 1 +1)

= \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) x 1 x 8

= \(\dfrac{1}{2}\) + \(\)\(\dfrac{1}{2}\) x 8

= \(\dfrac{1}{2}\) + 4

= \(\dfrac{9}{2}\) 

a; \(\dfrac{1}{4}\) + \(\dfrac{2}{5}\) + \(\dfrac{6}{8}\) + \(\dfrac{9}{15}\) + \(\dfrac{8}{1}\)

  = (\(\dfrac{1}{4}\) + \(\dfrac{6}{8}\)) + (\(\dfrac{2}{5}\) + \(\dfrac{9}{15}\)) + 8

= (\(\dfrac{1}{4}\) + \(\dfrac{3}{4}\)) + (\(\dfrac{2}{5}\) + \(\dfrac{3}{5}\)) + 8

= 1 + 1 + 8

= 2 + 8

= 10

b; \(\dfrac{1}{2}\) + \(\dfrac{2}{4}\) + \(\dfrac{3}{6}\) + \(\dfrac{4}{8}\) + \(\dfrac{5}{10}\) + \(\dfrac{6}{12}\) + \(\dfrac{7}{14}\) + \(\dfrac{8}{16}\) + \(\dfrac{9}{18}\) + \(\dfrac{10}{20}\)

= \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\)

= \(\dfrac{1}{2}\) x 10

= 5

a, 6/9+5/7+1/3=2/3+5/7+1/3=5/7+1=12/7

b, 17/7+6/5-20/14=17/7+6/5-10/7=6/5+1=11/5

c,2/5x1/4+3/4x2/5=2/5x(1/4+3/4)=2/5x1=2/5

d, 6/11:4/6+5/11:2/3=6/11:2/3+5/11:2/3=(6/11+5/11):2/3=3/2

nha

1dm3cm = 1,3dm

2dm47cm = 0,247 m

3m9cm = 3,09 m

536mm = 0,536 m

5km235m = 5,235km

4km74m = 4,074 km

3km3m = 3,003 km

2043m = 0,2043 km

chúc bạn được 10 điểm 

86x273-273x85+273

=273.(86-85+1)

=273.2

=546

bạn có cách trình bày nào khác ko 

mk làm tính nhiều các ý

1 và 9/10=1,9              2 và 66/100=2,66           3 và 72/100=3,72              4 và 999/1000=4,999

1,9;    2,66;      3,72;      4,999

Tiep

42,15km

500000000,6dm²

12 dm 44dm

7,4

7,500

4,123

12,3 m2=0,123 dam2

1,64 km =1 km 640 m=1640 m

3 tấn 4 tạ= 3,4 tấn

7 tấn 1/2 tạ=7,05 tấn

4 ha 356 dam2 = 7,56 ha

40 km 2150 m=42,15 km

5 km2 67 cm2 = 500 000 000,67dm2

12,44 m2=12 m2 44dm2

7,4 dm2=7,4 dm2

3/4 km2=7 500 dam2

412,3 m2 =4,123 dam2

Đương chéo còn lại của hình thoi là : 

\(\frac{9}{25}\times2:\frac{3}{5}=\frac{6}{5}m\)

Cạnh của hình thoi là :

\(\frac{4}{9}\times\left(\frac{3}{5}+\frac{6}{5}\right)=\frac{4}{5}m\)

Chu vi hình thoi là : 

\(\frac{4}{5}\times4=\frac{16}{5}m=320cm\)