1) a_n = a_1 + (n-1)d
a_212 = 0.3 + (212-1)0.1
a_212 = 0.3 + 21*0.1
a_212 = 0.3 + 2.1
a_212 = 2.4

Ответ: a_212 = 2.4

2) a_36 = a_1 + (36-1)0.3
1.5 = a_1 + 35*0.3
1.5 = a_1 + 10.5
a_1 = 1.5 - 10.5
a_1 = -9

Ответ: a_1 = -9

3) a_n = a_1 + (n-1)d
a_5 = a_1 + 4d
a_8 = a_1 + 7d

a_5 + a_8 = 82
(a_1 + 4d) + (a_1 + 7d) = 82
2a_1 + 11d = 82

a_3 = a_1 + 2d
a_9 = a_1 + 8d

a_3 + a_9 = 76
(a_1 + 2d) + (a_1 + 8d) = 76
2a_1 + 10d = 76

Now we have a system of linear equations:
2a_1 + 11d = 82
2a_1 + 10d = 76

Subtract the second equation from the first:
2a_1 + 11d - 2a_1 - 10d = 82 - 76
d = 6

Substitute d back into one of the equations:
2a_1 + 11(6) = 82
2a_1 + 66 = 82
2a_1 = 16
a_1 = 8

Ответ: a_1 = 8, d = 6