f(n)=n²cos(nπ)= (-1)^n*n²（当n为奇数时cos(nπ)=-1；n为偶数时cos(nπ)=1）an=f(n)+f(n+1)= (-1)^n*n² + (-1)^(n+1)* (n+1)²= (-1)^(n+1)*((n+1)² -n²)=(-1)^(n+1)*(2n+1)a1+a...