Explanation:
21.
1, 2, 4, 8, 16, 31, ... , ....
by using the formula, we can find :
T7 = [7⁴ - 6(7)³ +23(7)² - 18(7) +24] / 24 =
(2401 - 2058 + 1127 - 126 +24) / 24 =
1368/ 24 = 57
T8 = [8⁴-6(8)³+23(8)²-18(8)+24]/24 =
(4096-3072+1472-144+24)/24 =
2376/ 24 = 99