很简单。28+99x72=28+100x72-72=28+7200-72=7128+100-72=7128+28=7156
28+99×72＝28+7200-72＝7100+128-72＝7100+56＝7156
28+99×72=28+（100–1）×72=28+7200–72=7200–44=7156
