Trong phân tử $CO{\left(N{H}^2\right)}^2$ có 2 nguyên tử N.

Trong phân tử ${\left(N{H}^4\right)}^{2}S{O}^4$ có 2 nguyên tử N.

Trong phân tử $N{H}^{4}N{O}^3$ có 2 nguyên tử N.

Trong phân tử $Ca{\left(N{O}^3\right)}^2$ có 2 nguyên tử N.

Vậy trong cùng 1 phân tử, các chất trên đều cùng số nguyên tử N, vậy nên bác nông dân chọn loại nào cũng được.

a)SO₃

b)CH₄

c)Fe₂S₃

(1) nguyên tử

(2)nguyên tố

(3)1:2

(4) gấp khúc

(5) đường thẳng

a)\(-1,62+\dfrac{2}{5}+x=7\)

                          \(\dfrac{2}{5}+x=7-\left(-1,62\right)\)

                         \(\dfrac{2}{5}+x=7+1,62\)

                         \(\dfrac{2}{5}+x=8,62\)

                                \(x=8,62+\dfrac{2}{5}\)

                                \(x=\dfrac{431}{50}+\dfrac{2}{5}\)

                               \(x=\dfrac{451}{50}\)

a) P=\(\dfrac{2}{3}+\dfrac{1}{4}+\dfrac{3}{5}-\dfrac{7}{45}+\dfrac{5}{9}+\dfrac{1}{12}+\dfrac{1}{35}\)

   P=\(\left(\dfrac{2}{3}+\dfrac{1}{4}+\dfrac{1}{12}\right)+\left(\dfrac{5}{9}-\dfrac{7}{45}\right)+\dfrac{3}{5}+\dfrac{1}{35}\)

   P=\(1+\dfrac{4}{5}+\dfrac{3}{5}+\dfrac{1}{35}\)

  P= \(2\dfrac{1}{35}\)

b)Q=\(\left(5-\dfrac{3}{4}+\dfrac{1}{5}\right)-\left(6+\dfrac{7}{4}-\dfrac{8}{5}\right)-\left(2-\dfrac{5}{4}+\dfrac{16}{5}\right)\)

Q=                      \(\left(5-6-2\right)+\left(-\dfrac{3}{4}-\dfrac{7}{4}+\dfrac{5}{4}\right)+\left(\dfrac{1}{5}-\dfrac{8}{5}-\dfrac{16}{5}\right)\)

Q= \(-\left(3+\dfrac{5}{4}+\dfrac{23}{5}\right)\)

Q=\(\left(3+1\dfrac{1}{4}+4\dfrac{3}{5}\right)\)

Q=\(-8\dfrac{17}{20}\)

Dọc theo chiều dài, ta trồng được:

5,5 \(\div\dfrac{1}{4}\)= 22 ( khóm hoa)

Dọc theo chiều rộng, ta trồng được:

3,75\(\div\dfrac{1}{4}\)= 15 ( khóm hoa)

Như vậy, số khóm hoa trồng được dọc theo hai cạnh của mạnh vườn: 

$(22+15).2-4=70\; (\; khóm\; hoa)$

 a)\(\dfrac{1}{5}+\dfrac{4}{5}\div\text{x}=\dfrac{3}{4}\)

            \(\dfrac{4}{5}\div\text{x}=\dfrac{3}{4}-\dfrac{1}{5}\)

            \(\dfrac{4}{5}\div\text{x}=\dfrac{11}{20}\)

                   \(\text{x}=\dfrac{11}{20}\div\dfrac{4}{5}\)

                   \(\text{x}=\dfrac{11}{20}\times\dfrac{5}{4}\)

                   \(\text{x}=\dfrac{55}{80}\)

                   \(\text{x}=\dfrac{11}{16}\)

        Vậy x= \(\dfrac{11}{16}\)

b) x\(+\dfrac{1}{2}=1-\text{x}\)

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

           \(2\text{x}=\dfrac{1}{2}\)

             \(\text{x}=\dfrac{1}{2}\div2\)

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

      Vậy x = \(\dfrac{1}{4}\)

a) A=\(\dfrac{3}{5}\times\dfrac{6}{7}+\dfrac{3}{7}\times\dfrac{3}{5}-\dfrac{2}{7}\times\dfrac{3}{5}\)

    A=\(\dfrac{3}{5}\times(\dfrac{6}{7}+\dfrac{3}{7}-\dfrac{2}{7})\) 

    A=\(\dfrac{3}{5}\times1\)

    A=\(\dfrac{3}{5}\)

 b)B=\((-13\times\dfrac{2}{5}+\dfrac{-2}{9}\div\dfrac{5}{2}+\dfrac{2}{5}\times\dfrac{11}{9})\times\dfrac{5}{2}\)

    B=\((-13\times\dfrac{2}{5}+\dfrac{-2}{9}\times\dfrac{2}{5}+\dfrac{2}{5}\times\dfrac{11}{9})\times\dfrac{5}{2}\)

     B=\(\dfrac{2}{5}\times(-13+\dfrac{-2}{9}+\dfrac{11}{9})\times\dfrac{5}{2}\)

     B=\(\dfrac{2}{5}\times-12\times\dfrac{5}{2}\)

     B=\(\dfrac{-24}{5}\times\dfrac{5}{2}\)

     B=\(-12\)

c) C= \(\dfrac{-4}{5}+\dfrac{5}{7}\times\dfrac{3}{2}+\dfrac{-1}{5}+\dfrac{2}{7}\times\dfrac{3}{2}\)

           C=\(\dfrac{3}{2}\times(\dfrac{-4}{5}+\dfrac{-1}{5})+(\dfrac{5}{7}+\dfrac{2}{7})\)

           C=\(\dfrac{3}{2}\times(-1)+1\)

          C=\(\dfrac{-3}{2}+1\)

          C=\(\dfrac{-1}{2}\)

d)D=\(\dfrac{4}{9}\div(\dfrac{1}{15}-\dfrac{2}{3})+\dfrac{4}{9}\div(\dfrac{1}{11}-\dfrac{5}{22})\)

           D=\(\dfrac{4}{9}\div(\dfrac{-4}{15}-\dfrac{3}{12})+\dfrac{4}{9}\div(\dfrac{-16-15}{60})\)

           D=\(\dfrac{4}{9}\times\dfrac{-60}{31}\)

           D=\(-\dfrac{4}{9}\times\dfrac{60}{31}\)

           D=\(\dfrac{4\times20}{3\times31}\)

           D=\(-\dfrac{80}{93}\)

a) A= \(\dfrac{2}{7}\times(\dfrac{1}{4}-\dfrac{1}{3})\div\dfrac{2}{7}\times(\dfrac{3}{9}-\dfrac{2}{5})\)

    A= \(\dfrac{2}{7}\times\dfrac{7}{12}\div\dfrac{2}{7}\times\dfrac{-1}{15}\)

    A= \(\dfrac{1}{6}\div\dfrac{-2}{105}\)

    A= \(\dfrac{-35}{4}\)

b) B= \(\dfrac{(\dfrac{1}{5}-\dfrac{2}{7})\times\dfrac{3}{4}-\dfrac{3}{4}(\dfrac{1}{3}-\dfrac{2}{7})}{\dfrac{1}{5}\times\dfrac{2}{7}-\dfrac{1}{3}(\dfrac{2}{7}+\dfrac{3}{9})+\dfrac{3}{9}\times\dfrac{1}{5}}\)

     B=\(\dfrac{\dfrac{3}{4}(\dfrac{1}{5}-\dfrac{2}{7}-\dfrac{1}{3}+\dfrac{2}{7})}{\dfrac{1}{5}\times\dfrac{2}{7}-\dfrac{1}{3}\times\dfrac{2}{7}+\dfrac{1}{3}\times\dfrac{3}{9}+\dfrac{3}{9}\times\dfrac{1}{5}}\)

     B= \(\dfrac{\dfrac{3}{4}[(\dfrac{1}{5}-\dfrac{1}{3})-(\dfrac{2}{7}-\dfrac{2}{7})]}{\dfrac{2}{7}(\dfrac{1}{5}-\dfrac{1}{3})+\dfrac{3}{9}(\dfrac{1}{3}-\dfrac{1}{5})}\)

     B=\(\dfrac{\dfrac{3}{4}(-\dfrac{2}{15}+\dfrac{1}{7})}{\dfrac{2}{7}\times(\dfrac{-2}{15})+\dfrac{3}{9}\times(\dfrac{2}{5})}\)

     B= \(\dfrac{\dfrac{3}{4}\times(\dfrac{-2}{15})}{\dfrac{2}{7}(-\dfrac{2}{15})+\dfrac{3}{9}\times\dfrac{2}{15}}\)

     B=\(\dfrac{\dfrac{3}{4}\times(\dfrac{-2}{15})}{\dfrac{2}{7}(\dfrac{-2}{15})+\dfrac{1}{3}\times\dfrac{2}{15}}\)

     B=\(\dfrac{\dfrac{3}{4}}{(\dfrac{2}{7}-\dfrac{1}{3})}\)

     B=\(\dfrac{\dfrac{3}{4}}{\dfrac{-1}{21}}\)

     B=\(\dfrac{3}{4}\times(\dfrac{-21}{1})\)

     B=\(\dfrac{63}{4}\)