q^3=a5/a2=32/4=8
q=2
a2=a1q
4=a1*2
a1=2
an=a1*q^(n-1)
=2*2^(n-1)
=2^n
a2n=2^2n
a2*1=4
a2*2=16
a2*3=64
Cn=1/2n(n+1) +a2n
=1/2*1*(1+1)+4+1/2*2*(2+1)+16+.+1/2n(n+1) +2^2n
=1/2[1/1*2+1/2*3+...+1/n(n+1)]+4+16+64+..+2^2n
=1/2[1-1/2+1/2-1/3+1/3-1/4+...+1/n-1/(n+1)]+4*(1-4^n)/(1-4)
=1/2*[1-1/(n+1)]+4*(4^n-1)/3
=n/2(n+1)+4*(4^n-1)/3