1. a+b b+a ax(bxc) (axb)xc axc+bxc (a+b)xc 2. 41+216+159 =(41+159)+216 =200+216 =416 45x16 =45x2x8 =90x8 =720 12x13x5 =12x5x13 =60x13 =780 230-76-124 =230-(76+124) =230-200 =30 19+99x19 =19x(1+99) =19x100 =1900 256x9－56x

第一题有错的怎么会有两个一样的（a×b）×c ?