[tex]\displaystyle\bf\\a_{1} =12\\\\d=-2\\\\\\a_{6} =a_{1}+5d=12+5\cdot(-2)=12-10=2\\\\\boxed{a_{6}=2} \\\\\\S_{20} =\frac{2a_{1}+19d }{2}\cdot 20=(2a_{1} +19d)\cdot 10=\\\\\\=\Big(2\cdot 12+19\cdot(-2)\Big)\cdot 10=(24-38)\cdot 10=-14\cdot 10=-140\\\\\\\boxed{S_{20} =-140}[/tex]
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[tex]\displaystyle\bf\\a_{1} =12\\\\d=-2\\\\\\a_{6} =a_{1}+5d=12+5\cdot(-2)=12-10=2\\\\\boxed{a_{6}=2} \\\\\\S_{20} =\frac{2a_{1}+19d }{2}\cdot 20=(2a_{1} +19d)\cdot 10=\\\\\\=\Big(2\cdot 12+19\cdot(-2)\Big)\cdot 10=(24-38)\cdot 10=-14\cdot 10=-140\\\\\\\boxed{S_{20} =-140}[/tex]