\[\begin{cases} { x }^{ 2 }+{ y }^{ 2 }=30 \\ x+y=10 \end{cases}\]
If the above equations are true simultaneously, then find the value of $xy$.
\(x^2+y^2+2xy=30+2xy\\ (x+y)^2=30+2xy\\ \)
You can finish it.
