Bài 3:
a) Áp dụng t/c dtsbn:
\(\dfrac{x}{2}=\dfrac{y}{5}=\dfrac{y-x}{5-2}=\dfrac{21}{3}=7\)
\(\Rightarrow\left\{{}\begin{matrix}x=7.2=14\\y=7.5=35\end{matrix}\right.\)
b) Áp dụng t/c dtsbn:
\(\dfrac{x}{3}=\dfrac{y}{7}=\dfrac{x+y}{3+7}=\dfrac{25}{10}=\dfrac{5}{2}\)
\(\Rightarrow\left\{{}\begin{matrix}x=\dfrac{5}{2}.3=\dfrac{15}{2}\\y=\dfrac{5}{2}.7=\dfrac{35}{2}\end{matrix}\right.\)
Bài 4:
Gọi độ dài 3 cạnh là x,y,z(cm)(a,b,c>0)
Nửa chu vi là: \(45:2=\dfrac{45}{2}\left(cm\right)\)
Áp dụng t/c dtsbn:
\(\dfrac{x}{4}=\dfrac{y}{5}=\dfrac{z}{6}=\dfrac{x+y+z}{4+5+6}=\dfrac{\dfrac{45}{2}}{15}=\dfrac{3}{2}\)
\(\Rightarrow\left\{{}\begin{matrix}x=\dfrac{3}{2}.4=6\left(cm\right)\\y=\dfrac{3}{2}.5=\dfrac{15}{2}\left(cm\right)\\z=\dfrac{3}{2}.6=9\left(cm\right)\end{matrix}\right.\)
Bài 5:
Gọi số học giỏi, khá, tb là a,b,c(em)(a,b,c∈N*)
Áp dụng t/c dtsbn:
\(\dfrac{a}{2}=\dfrac{b}{3}=\dfrac{c}{5}=\dfrac{b+c-a}{5+3-2}=\dfrac{180}{6}=30\)
\(\Rightarrow\left\{{}\begin{matrix}a=30.2=60\\b=30.3=90\\z=30.5=150\end{matrix}\right.\)
