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a+ 4b + a + 7b = 29
2a + 11 b = 29
U3 = 18
a + 2b = 18
a + 2b = 18 | x 2 | 2a + 4b = 36
2a + 11b = 29 | x 1 | 2a + 11b = 29
----------------------- -
- 7 b = 7
b = -1
a + 2b = 18
a+ 2( -1) = 18
a - 2 = 18
a= 18 + 2
a = 20
U31 = a + ( n - 1) b
= 20 + 30.(-1)
= 20 - 30
= -10