Question

в арифметической прогрессии а7=20 а10=11 найдите а19

Answer 1

d=a10-a7=-9/3= -3 a19=a10+(-3)*8 a19=-13

Answer 2

a7=20   a10=11  a19-?
a19=a1+18d

a7=a1+6d
a10=a1+9d
20=a1+6d
11=a1+9d
9=-3d
3d=-9
d=-3
a7=a1+6d
20=a1+6*(-3)
20=a1-18
a1=-18-20
-a1=-38
a1=38
a19=38+18*(-3)
a19=38-54
a19=-16