Observe the numbers of the following groups, find out their rules, and express the nth (n is a positive integer) number with the formula: Such as: 1,4,9,16,25 The number of n is n & # 178; (1) - 2, - 8, - 32, - 128 ；（2）-2,4,-8,16,-32,… ；（3）-1,4,-9,16,-25,… .

Observe the numbers of the following groups, find out their rules, and express the nth (n is a positive integer) number with the formula: Such as: 1,4,9,16,25 The number of n is n & # 178; (1) - 2, - 8, - 32, - 128 ；（2）-2,4,-8,16,-32,… ；（3）-1,4,-9,16,-25,… .

1, the latter number is 4 times of the former number, the N-1 power of 2 * 4
2, (- 2) to the power of n
3, n power of (- 1) times 2 power of n

The general formula of 1 3 6 10 15

3=2+1,6=1+2+3,10=1+2+3+4,...
General term formula a (n) = 1 + 2 +... + n = {(1 + 2 +... + n) + [n + (n-1) +... + 1]} / 2
=(n+1)n/2

Four numbers 1.3.6.10, find the general formula of an

an=n(1+n)/2