D

\(a,\dfrac{8}{14}=\dfrac{x+1}{7}\\ \Leftrightarrow\dfrac{x+1}{7}=\dfrac{4}{7}\\ \Leftrightarrow x+1=4\\ \Leftrightarrow x=3\\ b,\dfrac{-4}{3}=\dfrac{5x+1}{-27}\\ \Leftrightarrow\dfrac{36}{-27}=\dfrac{5x+1}{-27}\\ \Leftrightarrow5x+1=36\\ \Leftrightarrow5x=35\\ \Leftrightarrow x=7\)

\(\dfrac{-21}{20}+\dfrac{4}{5}.\dfrac{7}{6}-\dfrac{3}{4}\\ =\dfrac{-21}{20}+\dfrac{14}{15}-\dfrac{3}{4}\\ =\dfrac{-63}{60}+\dfrac{56}{60}-\dfrac{45}{60}\\ =\dfrac{-52}{60}\\ =-\dfrac{13}{15}\)

\(\left(-x^2y\right)^3.\dfrac{1}{2}x^2y^3.\left(-2xy^2z\right)^3\\ =\left(-1\right)x^6y^3.\dfrac{1}{2}x^2y^3.\left(-8\right)x^3y^6z^3\\ =\left[\left(-1\right).\dfrac{1}{2}.\left(-8\right)\right]\left(x^6.x^2.x^3\right)\left(y^3.y^3.y^6\right).z^3\\ =4x^{11}y^{12}z^3\)

\(j,\dfrac{2}{5.7}+\dfrac{2}{7.9}+...+\dfrac{2}{53.55}=\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+...+\dfrac{1}{53}-\dfrac{1}{55}=\dfrac{1}{5}-\dfrac{1}{55}=\dfrac{11}{55}-\dfrac{1}{55}=\dfrac{10}{55}=\dfrac{2}{11}\\ k,\dfrac{1}{2}.\dfrac{2}{3}.\dfrac{3}{4}.\dfrac{4}{5}...\dfrac{99}{100}=\dfrac{1}{100}.\dfrac{2}{2}.\dfrac{3}{3}...\dfrac{99}{99}=\dfrac{1}{100}.1.1...1=\dfrac{1}{100}\)

d, `3,15+2,4=5,55`

e, \(\dfrac{5}{7}.\dfrac{2}{11}+\dfrac{5}{7}.\dfrac{9}{11}=\dfrac{5}{7}\left(\dfrac{2}{11}+\dfrac{9}{11}\right)=\dfrac{5}{7}.\dfrac{11}{11}=\dfrac{5}{7}.1=\dfrac{5}{7}\)

f, `1,25.3,6+3,6.8,75=3,6(1,25+8,75)=3,6.10=36`

\(h,\dfrac{1}{1.2}+\dfrac{1}{2.3}+...+\dfrac{1}{99.100}\\ =1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+...+\dfrac{1}{99}-\dfrac{1}{100}\\ =1-\dfrac{1}{100}\\ =\dfrac{99}{100}\)

3.A

4.B

5.A

6.B

7.C

8.B

khó wá, ko nghĩ ra .-.

bao h kể đi