[tex]\displaystyle\bf\\a_{1}=-3,6\\\\d=0,6\\\\S_{24} =?\\\\\\a_{24} =a_{1} +23d=-3,6+23\cdot 0,6=-3,6+13,8=10,2\\\\\\S_{24} =\frac{a_{1} + a_{24} }{2} \cdot 24=(a_{1} + a_{24} )\cdot 12=(-3,6+10,2)\cdot 12=\\\\\\=6,6\cdot 12=13,2\\\\\\Otvet \ : \ S_{24} =13,2[/tex]