Question

数列{an}的通项公式为：an=2的n次方-1 求数列{an}前n项和

Answer 1

an=2^n-1

a1=2-1

a2=2^2-1

a3=2^3-1

Sn=2-1+2^2-1+2^3-1+.+2^n-1

=2+2^2+2^3+...+2^n+1*n

=-2(1-2^n)+n

=2^(n+1)+n-2