Question

已知等比数列的前n项和Sn=4的n次方+a则a的值等于（） A.-4 B.-1 C.0 D.1

Answer 1

Sn=4^n+a (1)

put n=1

a1=4+a

S(n-1) = 4^(n-1) + a (2)

(1) -(2)

an = 3. 4^(n-1)

put n=1

4+a = 3

a= -1