[tex]\displaystyle\bf\\a_{1} =-4\\\\d=4\\\\a_{80} =a_{1} +79d=-4+79\cdot4=-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\cdot4=-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]