1/2^2+1/2^3+……+1/2^n
=1/4+1/8+……+1/2^n
=1/4+1/8+……+1/2^n+1/2^n-1/2^n
=1/4+1/8+……+1/2(n-1)-1/2^n
=……
=1/4+1/8+1/8-1/2^n
=1/4+1/4-1/2^n
=1/2-1/2^n