[tex]\displaystyle\bf\\a_{1} =4\\\\d=-4\\\\a_{80} =a_{1} +79d=4+79\cdot(-4)=4-316=-312\\\\\\S_{80} =\frac{a_{1}+a_{80} }{2} \cdot 80=(a_{1} +a_{80} )\cdot40=(4-312)\cdot 40=-308\cdot40=-12320[/tex]
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[tex]\displaystyle\bf\\a_{1} =4\\\\d=-4\\\\a_{80} =a_{1} +79d=4+79\cdot(-4)=4-316=-312\\\\\\S_{80} =\frac{a_{1}+a_{80} }{2} \cdot 80=(a_{1} +a_{80} )\cdot40=(4-312)\cdot 40=-308\cdot40=-12320[/tex]