a.
\(\frac{2x}{7}=\frac{3y}{2}\Rightarrow4x=21y\)
\(x-y=17\Rightarrow x=17+y\)
\(\Rightarrow4\left(17+y\right)=21y\Rightarrow68+4y=21y\Rightarrow17y=68\Rightarrow y=4\)
\(\Rightarrow x=17+y=17+4=21\)
b.
\(\frac{x}{2}=\frac{y}{5}\Rightarrow5x=2y\)
\(x\cdot y=40\Rightarrow x=\frac{40}{y}\)
\(\Rightarrow5\cdot\frac{40}{y}=2y\Rightarrow\frac{200}{y}=2y\Rightarrow2y^2=200\Rightarrow y=\pm10\)
\(\Rightarrow\orbr{\begin{cases}x=4\\x=-4\end{cases}}\)
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