an=5/2^n-7/（n^2+3n+2）=5/2^n-7/(n+1)(n+2)=5*(1/2)^n-7*[1/(n+1)-1/(n+2)]前n项和等于=5*【1/2+1/2^2+1/2^3+……1/2^n】-7*【1/2-1/3+1/3-1/4+1/4-1/5……+1/(n+1)-1/(n+2)】=5*【1-1/2^n】-7*【1/2-1/(n+2)】极...