[tex]\displaystyle\bf\\\left \{ {{a_{3} =-0,8} \atop {a_{11} =-5}} \right. \\\\\\-\left \{ {{a_{1} +2d=-0,8} \atop {a_{1} +10d=-5}} \right. \\----------\\-8d=4,2\\\\d=-0,525\\\\a_{1} =-0,8-2d=-0,8-2\cdot(-0,525)=-0,8+1,05=0,25\\\\a_{16} =a_{1}+15d=0,25+15\cdot(-0,525)=0,25-7,875=-7,625\\\\\\S_{16} =\frac{a_{1}+a_{16} }{2} \cdot16=(a_{1} +a_{16} )\cdot 8=(0,25-7,625)\cdot 8=-59\\\\\\Otvet:S_{16} =-59[/tex]