\(a.\dfrac{6}{5}=\dfrac{18}{x}\Rightarrow x=\dfrac{18\cdot5}{6}=15\\ \text{Vậy}\text{ }x=15.\)

\(b.\dfrac{3}{4}=\dfrac{-21}{x}\Rightarrow x=\dfrac{-21\cdot4}{3}=28\\ \text{ }\text{ }\text{ }\text{ }\text{Vậy }x=28.\)

\(c.\dfrac{x}{4}=\dfrac{21}{28}\Rightarrow x=\dfrac{21\cdot4}{28}=3\\ \text{Vậy }x=3.\)

\(d.\dfrac{-8}{2x}=\dfrac{3}{-9}\Rightarrow x=\dfrac{-8\cdot\left(-9\right)}{3}:2=12\\ \text{Vậy }x=12.\)

\(e.\dfrac{-4}{11}=\dfrac{x}{22}=\dfrac{40}{z}\\ \Rightarrow x=\dfrac{-4\cdot22}{11}=-8\\ \Rightarrow z=\dfrac{22\cdot40}{-8}=-110\\ \text{Vậy }x=-8;z=-110.\)

\(f.\dfrac{-3}{4}=\dfrac{x}{20}=\dfrac{21}{y}\\ \Rightarrow x=\dfrac{-3\cdot20}{4}=-15\\ \Rightarrow y=\dfrac{21\cdot20}{-15}=-28\\ \text{Vậy }x=-15;y=-28.\)

\(g.\dfrac{-4}{8}=\dfrac{x}{-10}=\dfrac{-7}{y}=\dfrac{z}{-24}\\ \Rightarrow x=\dfrac{-4\cdot\left(-10\right)}{8}=5\\ \Rightarrow y=\dfrac{-7\cdot\left(-10\right)}{5}=14\\ \Rightarrow z=\dfrac{-7\cdot\left(-24\right)}{14}=12\\ \text{Vậy }x=5;y=14;z=12.\)

\(h.\dfrac{x}{4}=\dfrac{9}{x}\\ \Rightarrow x\cdot x=9\cdot4\\ \Rightarrow x\cdot x=36\\ \Rightarrow x\cdot x=6\cdot6\\ \text{Vậy }\text{cả hai }x=6.\)

Bài giải

Chu vi hình H là:\(6\text{×}3,14=18,84\left(cm\right)\)

Đáp số: \(18,84cm.\)

Bài giải

Tích của bán kính của hình tròn là: \(12,56:3,14=4\left(cm\right)\)

Có tích bán kính = r x r, mà tích bán kính bằng 4 nên bán kính hình tròn bằng 2.

Chu vi hình tròn là: \(2\text{×}\left(2\text{×}3,14\right)=12,56\left(cm\right)\)

Đáp số: \(12,56cm.\)

\(2014\text{×}3+4028\text{×}2+2014\text{×}5-2014\text{×}2\\ =2014\text{×}3+2014\text{×}2\text{×}2+2014\text{×}5-2014\text{×}2\\ =2014\text{×}3+2014\text{×}4+2014\text{×}5-2014\text{×}2\\ =2014\text{×}\left(3+4+5-2\right)=2014\text{×}10=20140\)

\(A.5dam^239m^2=5,39dam^2\\ B.82dam^251m^2=82,51dam^2\\ C.8,6352m^2=8m^263dm^252cm^2\\ D.39m^28dm^2=39,08m^2\)

\(A.6m^25dm^2=6,05m^2\\ B.15,4km^2=15km^24000dam^2\\ C.3km180cm=3000,18m\\ D.7,469km^2=746ha9m^2\)