дано:a2=1;a5=13
найти:a11;d;S25

a2=a1+d =>   1=a1+d      => a1=d-1         d-1=4d-13 => -3d=-12=> d=4a5=a1+4d => 13=a1+4d  =>a1=4d-13a1=a2-d =>a1=-3a11=a1+(n-1)d => a11=-3+(10*4)=37a25=a1+24*d => a25=-3+(24*4)=93S25=((a1+a25)/2)*n =((-3=93)/2)*25=1125