题目

Today’s modern life discourages people from taking time to prepare their own meals. Eating in the car and grabbing a quick bite in a restaurant are ever more popular ways of dining. It takes a little extra effort and time to cook your own food, but it’s a worthwhile pursuit(追求) that can benefit your health, wallet and family. People across the country are waking up to the fact that the food offered by most fast food restaurants isn’t particularly healthy. While these restaurants have made attempts lately to add healthier meals to their menus, it’s still a far cry from a dish对正整数n，设曲线y=xn（1-x）在x=2处的切线与y轴交点的纵坐标为an，则数列的前n项和的公式是     ．