\(a,A=\left\{10;11;12;...;69;70\right\}\)

Số phần tử trong tập hợp A là:

\(\left(70-10\right):1+1=61\) (phần tử)

\(b,B=\left\{0;2;4;...;48;50\right\}\)

Số phần tử trong tập hợp B là:

\(\left(50-0\right):2+1=26\) (phần tử)

\(c,C=\left\{1;4;7;...;97;100\right\}\)

Số phần tử trong tập hợp C là:

\(\left(100-1\right):3+1=34\) (phần tử)

Ta biết: \(1kg=0,001\) tấn

\(\rightarrow85kg=0,085\) tấn

\(\Rightarrow D.0,085\) 

\(x\in B\left(3\right)=\left\{0;3;6;9;...\right\}\)

Mà \(3\le x\le9\) nên \(x=\left\{3;6;9\right\}\)

\(\dfrac{2017}{2008}=1+\dfrac{9}{2008}\)

\(\dfrac{2008}{1999}=1+\dfrac{9}{1999}\)

Vì: \(\dfrac{9}{2008}< \dfrac{9}{1999}\) nên \(\dfrac{2017}{2008}< \dfrac{2008}{1999}\)

\(5\cdot2^3+7^9:7^7-1^{2020}\\ =5\cdot8+7^2-1\\ =40+49-1\\ =89-1\\ =88\)

\(5a^2-5=5\cdot a^2-5\cdot1=5\cdot\left(a^2-1\right)=5\cdot\left(a-1\right)\left(a+1\right)\)

\(a,\dfrac{2}{7}+\dfrac{5}{7}\cdot\dfrac{14}{25}=\dfrac{2}{7}+\dfrac{2}{5}=2\left(\dfrac{1}{7}+\dfrac{1}{5}\right)=2\cdot\dfrac{12}{35}=\dfrac{24}{35}\\ b,\dfrac{4}{7}:\left(\dfrac{2}{5}\cdot\dfrac{4}{7}\right)=\dfrac{4}{7}:\dfrac{2}{5}:\dfrac{4}{7}=\left(\dfrac{4}{7}:\dfrac{4}{7}\right):\dfrac{2}{5}=1:\dfrac{2}{5}=\dfrac{5}{2}\)

Số hạng là:

\(\left(100-1\right):3+1=34\) (số hạng)

Tổng là:

\(\left(100+1\right)\cdot34:2=1717\)

Vậy: \(100+97+94+...+4+1=1717\)

\(\dfrac{19}{37}+\left(\dfrac{1}{2}-\dfrac{19}{37}\right)\\ =\dfrac{19}{37}+\dfrac{1}{2}-\dfrac{19}{37}\\ =\left(\dfrac{19}{37}-\dfrac{19}{37}\right)+\dfrac{1}{2}\\ =0+\dfrac{1}{2}\\ =\dfrac{1}{2}\)

\(-\dfrac{7}{22}\cdot\dfrac{5}{3}+\dfrac{-7}{22}\cdot\dfrac{1}{3}-\left(-\dfrac{7}{22}\right)\cdot\dfrac{11}{21}\\=-\dfrac{7}{22}\cdot\dfrac{5}{3}-\dfrac{7}{22}\cdot\dfrac{1}{3}-\dfrac{7}{22}\cdot\dfrac{-11}{21}\\ =-\dfrac{7}{22}.\left(\dfrac{5}{3}+\dfrac{1}{3}-\dfrac{11}{21}\right)\\ =-\dfrac{7}{22}\cdot\dfrac{31}{21}\\ =-\dfrac{31}{66}\)