\left\{ \begin{array} { l } { a _ { 4 } + a _ { 7 } = 2 } \\ { a _ { 4 } \cdot a _ { 7 } = - 8 } \end{array} \right.
Solve for a_4, a_7
a_{4}=4\text{, }a_{7}=-2
a_{4}=-2\text{, }a_{7}=4
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