3/2,17/4,57/8,161/16.轉(zhuǎn)化一下,1+1/2,4+1/4,7+1/8,10+1/16.可以看出該數(shù)列由一個(gè)等差數(shù)列+一個(gè)等比數(shù)列組成的.
等差數(shù)列：1,4,7,10,.d=3,a1=1,則an=a1+（n-1）d=3n-2,Sn=na1+n(n-1)d/2 =(3n^2)/2-n/2
等比數(shù)列：1/2,1/4,1/8,1/16,.q=1/2,a1=1/2,則an=a1q^((n-1))=〖1/2〗^n,Sn=(a1(1-q^n))/(1-q)=1-〖1/2〗^n
所以所求數(shù)列通項(xiàng)式為An=3n-2+〖1/2〗^n,前n項(xiàng)和為Sn=(3n^2)/2-n/2+1-〖1/2〗^n