A = \(x^2-y^2=\left(x-y\right)\left(x+y\right)=7\left(x+y\right)\)
Ta có: \(x-y=7\)
\(\Rightarrow\left(x-y\right)^2=49\)
\(\Leftrightarrow x^2-2xy+y^2=49\)
\(\Leftrightarrow\left(x^2+2xy+y^2\right)-4xy=49\)
\(\Leftrightarrow\left(x+y\right)^2=49+4\cdot60=289\)
\(\Leftrightarrow\left[{}\begin{matrix}x+y=17\\x+y=-17\end{matrix}\right.\)
=> \(\left[{}\begin{matrix}A=7\cdot17=119\\A=7\cdot\left(-17\right)=-119\end{matrix}\right.\)
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