Verified answer

4)

a(n) = a1 + d*(n - 1)

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a(1) + a(10) = 12

a(8) - a(5) = 4

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a(1) + a(1) + 9d = 12

a(1) + 7d - (a(1) + 4d) = 4

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2a(1) + 9d = 12

3d = 4

d = 4/3

2*a(1) + 9*4/3 = 12

2a(1) + 12 = 12

a(1) = 0

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5)

x(n) = 3 + x(n - 1)

x(1) = 2

подставляем вместо х значение 2, потом 3 4 и 5

х(n) и x(n - 1) при n = 2 это x(2) и x(2 - 1) = x(1)

x(2) = 3 + x(1) = 3 + 2 = 5

x(3) = 3 + x(2) = 3 + 5 = 8

x(4) = 3 + x(3) = 3 + 8 = 11

x(5) = 3 + x(4) = 3 + 11 = 14