Арифметическая прогрессия,, 1) a1=-3, a2=-1 ,a7=? 2)a5=4,a8=13,a2=? 3)a2=5,a9=9,a4=?

1) а2=а1+d
d=a2-a1=-1+3=2
an=a1+(n-1)*d
a7=-3+6*2=9

2) an=a1+(n-1)*d
a5=a1+4*d
a8=a1+7*d
a1+4*d=4    (1)
a1+7*d=13  (2)
(2)-(1):
3*d=9
d=3
(1)→a1=4-4*d=4-4*3=-8
a2=a1+1*d=-8+3=-5

3) an=a1+(n-1)*d
a2=a1+1*d
a9=a1+8*d
a1+1*d=5    (1)
a1+8*d=9  (2)
(2)-(1):
7*d=4
d=4/7
(1)→a1=5-d=5-4/7=31/7
a4=a1+3*d=37/7+3*4/7=49/7=7