1.求数列1,x+x^2,x^2+x^3+x^4,x^3+x^4+x^5+x^6,...前n项之和
2.数列{an}中满足a1=1/2,a1+a2+...+an=n^2an,求an
3.数列{an}中,a1=1,an*an+1=4^n,求前n项和sn
4.f(x)=a1x+a2x^2+...+anx^n,且数列{an}为等差数列,n为正偶数,又f(1)=n^2,f(-1)=n,求证:f(1/2)