(1/2)×a1+(1/2²)×a2+...+(1/2ⁿ)×an=2n+5 (1)
(1/2)×a1+(1/2²)×a2+...+[1/2^(n-1)]×a(n-1)=2(n-1)+5 (2)
(1)-(2)
(1/2ⁿ)×an=2n+5-[2(n-1)+5]
an/2ⁿ=2
an=2^(n+1)
(1/2)×a1+(1/2²)×a2+...+(1/2ⁿ)×an=2n+5 (1)
(1/2)×a1+(1/2²)×a2+...+[1/2^(n-1)]×a(n-1)=2(n-1)+5 (2)
(1)-(2)
(1/2ⁿ)×an=2n+5-[2(n-1)+5]
an/2ⁿ=2
an=2^(n+1)