3a＾6b＾x-y与-5a＾xb＾3的和仍为单项式
则6=x,x-y=3
所以x=6,y=9
1/36x＾4-1/18xy＾2+1/9y＾3
=1/36*6^4-1/18*6*9^2+1/9*9^2
=36-27+81
=90