Sequence there !

Algebra Level 3

What is the sum of first 25 terms of the sequence where nthn^{th} term is defined by the formula

an=n22n1a_n = n^2 -2n -1

Details and assumptions

Sequence starts with a1a_1 , and it has a value a1=121=2a_1 = 1-2-1=-2

a2=222×21=1a_2 = 2^2-2\times 2 -1 = -1

a3=323×21=2a_3 = 3^2 - 3\times 2 -1 = 2

a4=424×21=7a_4 = 4^2-4\times 2 -1 =7 . . . .

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