Define S(n) = (20 + 21+ .......+(19+n))

and, U(n) = (20 + 19+ .......+ (21-n))

Then, the terms of the sequence T(n) is:

T(n)

= n^2 + S(n), where n is even

= n^2 + U(n), where n is odd

For example, T(2) would be 2^2+ 20+21 = 45, while T(3) would be 3^2+ 20+19+18 = 66

Consequently, the missing term is:

T(7)

= (20 + 19+...+14) + 7^2

= 119 + 49

= 168