令s=1+1/2+1/2²+……+1/2^100两边乘以22s=2+1+1/2+1/2²+……+1/2^99相减,中间相同的抵消2s-s=s=2-1/2^100所以1+1/2+1/2²+……+1/2^100=2-1/2^100
2+2²+2³+....+2^10设S=2+2²+2³+....+2^10那么2S=2²+2³+。。。+2^10+2^11两式相减可得S=2^11-2所以2+2²+2³+....+2^10=2^11-2