观察下列一串单项式的特点:xy,-2x^2y,4x^3y,-8x^4y,16x^5y,-32x ^6y,第n个单项式为多少,

问题描述:

观察下列一串单项式的特点:xy,-2x^2y,4x^3y,-8x^4y,16x^5y,-32x ^6y,第n个单项式为多少,
系数和次数是多少