（x-y)的十次方除以（y-x)的五次方除以（x-y）
=(x-y)^10÷(y-x)^5÷(x-y)
=(x-y)^10÷[-(x-y)^5]÷(x-y)
=-(x-y)^5÷(x-y)
=-(x-y)^4
同底数幂相除,底数不变,指数相减