Sn=1-3+5-7+…+(-1)^n-1(2n-1),则s15+s30=

问题描述:

Sn=1-3+5-7+…+(-1)^n-1(2n-1),则s15+s30=

答:Sn=1-3+5-7+.+[(-1)^(n-1)]*(2n-1)S15=1-3+5-7+.+25-27+29=(-2)+(-2)+...+(-2)+29 (14/2=7个-2)=-2*7+29=15S30=1-3+5-7+.+57-59=(-2)+(-2)+...+(-2) (30/2=15个-2)=-2*15=-30所以:S15+S30=15-30=-15所以:S1...