y×y的n+1次方—2y的n次方×y的2次方+y的n-1次方×y的3次方=y的n+2次方-2y的n+2次方+y的n+2次方=y的n+2次方-y的n+2次方=0
y·y^n+1－2y^n·y²+y^n-1·y³=y^n+2-2y^n+2+y^n+2=0
