Verified answer

[tex]\displaystyle\bf\\a_{1} =-4,7\\\\a_{2}=-4,3\\\\a_{2} =a_{1} +d\\\\d=a_{2}-a_{1} =-4,3-(-4.7)=-4,3+4,7=0,4\\\\a_{n} =a_{1} +d\cdot(n-1)\\\\a_{n} < 0\\\\a_{1} +d\cdot(n-1) < 0\\\\-4,7+0,4n-0,4 < 0\\\\0,4n < 5,1\\\\n < 12,75[/tex]

В этой прогрессии 12 отрицательных членов .

[tex]\displaystyle\bf\\a_{12} =a_{1} +11d=-4,7+11\cdot 0,4=-4,7+4,4=-0,3\\\\\\S_{12} =\frac{a_{1} + a_{12} }{2} \cdot 12=\Big(a_{1} +a_{12} \Big)\cdot 6=\Big(-4,7-0,3\Big)\cdot 6=\\\\\\=-5\cdot 6=-30\\\\Otvet \ : \ -30[/tex]