Ogólny wzór ciągu arytmetycznego [tex]a_n=a_1+(n-1)r[/tex]
[tex]a_3=6\\a_5=-4\\\\\left \{ {6=a_1+(3-1)r} \atop {-4=a_1+(5-1)r}} \right. \\\left \{ {6=a_1+2r} \atop {-4=a_1+4r}} \right. \\\\a_1=6-2r\\-4=6-2r+4r\\-10=2r\\r=-5\\a=6-2*(-5)=6+10=16\\\\\left \{ {{r=-5} \atop {a_1=16}} \right. \\\\S_1_2_3=16+(123-1)*(-5)=16+122*(-5)=16-610=-594\\S_1_2_0=16+(120-1)*(-5)=16+119*(-5)=16-595=-579\\\\S_1_2_3-S_1_2_0=-594-(-579)=-594+579=-15[/tex]
Jest to 3r=-15
