to mingja:

elling

.an=（－1）^(1+k)/2^k, k=1,2,3……问这些数和的范围


我觉得这题的和的极限是1/3， 你怎么算范围的？[em04]

kitten369

is the question an=（－1）^(1+n)/2^n, n=1,2,3……

a1= 1/2
a2= -1/4
a3=1/8

等比数列, 很容易的

in common Most Americans don't to how to sum 数列, So the answer will not be very specific. if you fogot the formula, just guess,you still could solve it.