【正确答案】利用Jaccobi恒等式[[A，B]，C]+[[B，C]，A]+[[C，A]，B]=0得
   [[▽²,x^ly^mzⁿ]，r²]=-[[x^ly^mzⁿ，r²]，▽²]-[[r²，▽²]，x^ly^mzⁿ]
   =-[[r²，▽²]，x^ly^mzⁿ]=-[▽·[r²，▽]+[r²，▽]·▽，x^ly^mzⁿ]
   =2[▽·r+r·▽，x^ly^mzⁿ]=2[▽，x^ly^mzⁿ]·r+2r·[▽，x^ly^mzⁿ]
   =2(lx^l-1y^mzⁿex+mx^ly^m-1zⁿe_x+nx^ly^mz^n-1e_x)·r+2r·(lx^l-1y^mzⁿe_x+mx^ly^l-1zⁿe_x+nx^lymz^n-1e_x)
   =4(l+m+n)x^ly^mzⁿ