а4=а1+3d a1+3d=10 a1=10-3d
a7=a1+6d a1+6d=19 10-3d+6d=19 3d=9 d=3 a1=10-3*3=1
s=(2*1+9*3)*10/2=29*5= 145
а4=а1+3d a1+3d=10 a1=10-3d
a7=a1+6d a1+6d=19 10-3d+6d=19 3d=9 d=3 a1=10-3*3=1
s=(2*1+9*3)*10/2=29*5= 145