an=Sn-S(n-1)=S(n-1)+2^n
Sn=2S(n-1)+2^n
兩邊除以2^n
Sn/2^n=S(n-1)/2^(n-1)+1
所以Sn/2^n等差,d=1
S1/2^1=3/2
Sn/2^n=n+1/2
Sn=(2n+1)*2^(n-1)
an=
3,n=1
Sn-S(n-1)=(n+3/2)*2^(n-1),n≥2怎么似懂非懂啊 鄙人駑鈍1/n的前N項(xiàng)和小于2具體點(diǎn)啊