Question

Find the explicit formula for the following sequence.
15, 29, 49, 75, 107,....

Answer 1

Explanation:

We are given the sequence

`15, 29, 49, 75, 107\\15->29 = 14\\29 -> 49 = 20\\49->75 = 26\\75 -> 107 = 32\\14, 20, 26, 32\\14 -> 20= 6\\20 -> 26 = 6\\26 -> 32 = 6\\\\6, 6, 6\\6->6 = 0`

Since we had to find the differences twice, it is a second degree polynomial

`y = an^2 + bn + c\\plugging \: in \: n = 1, 2, 3...\\the \: sequence \: is \\a+b+c, 4a + 2b + c, 9a + 3b + c...\\the \: diff \: under \: is \\3a + b, 5a + b, 7a + b...\\the \: diff \: under \: is \\2a, 2a, 2a...\\so, \\2a = 6\\a = 3\\3(3) + b = 14\\b = 5\\\\a +b + c = 15\\3 + 5 + c = 15\\c = 7\\`

so,

`y = 3n^2 + 5n + 7`