The first two terms of the sequence are 3 and 12 and the 3rd and 4th terms are 25 and 42.
The 3rd and 4th terms of the sequence is determined from second level difference as shown below;
(c - b) - (b - a) = 4 ----(1)
(d - c) - (c - b) = 4 ---- (2)
from (1);
(c - 12) - (12 - 3) = 4
(c - 12) - (9) = 4
c - 21 = 4
c = 25
from (2);
(d - 25) - (25 - 12) = 4
d - 38 = 4
d = 42
The sequence = 3, 12, 25, 42
check
first difference = (12 - 3), (25-12), (42-25) = 9, 13, 17
second difference = (13 - 9), (17 - 13) = 4, 4
Thus, the first two terms of the sequence are 3 and 12 and the 3rd and 4th terms are 25 and 42.
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