Câu 5:
\(5x=-4y\Rightarrow\dfrac{x}{-4}=\dfrac{y}{5}\)
Áp dụng t/c dtsbn:
\(\dfrac{x}{-4}=\dfrac{y}{5}=\dfrac{2x}{-8}=\dfrac{2y}{10}=\dfrac{2x+2y}{-8+10}=\dfrac{7}{2}\)
\(\Rightarrow\left\{{}\begin{matrix}x=\dfrac{7}{2}.\left(-4\right)=-14\\y=\dfrac{7}{2}.5=\dfrac{35}{2}\end{matrix}\right.\)
Câu 6:
Xét tam giác ABC có:
\(\left\{{}\begin{matrix}\widehat{A}=180^0-\left(\widehat{B}+\widehat{C}\right)\\\widehat{A}=180^0-3\widehat{C}\end{matrix}\right.\)\(\Rightarrow\widehat{B}+\widehat{C}=3\widehat{C}\Rightarrow\widehat{B}=2\widehat{C}\left(đpcm\right)\)
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