首先-1.8的3/5次方为负数，其余的两个为正数，则-1.8的3/5次方最小
然后3.8的-2/3次方=1/(3.8的2/3次方),3.8的2/3次方>1的2/3次方=1
那么3.8的-2/3次方=1/(3.8的2/3次方）<1
最后3.9的2/5次方>1的2/5次方=1
所以3.9的2/5次方>3.8的-2/3次方>-1.8的3/5次方
(x+y)的5次方-(x+y)的2次方
=(x+y)²[(x+y)³-1]
=(x+y)²(x+y-1)[(x+y)²+(x+y)+1]
=(x+y)²(x+y-1)(x²+y²+2xy+x+y+1)
