892×57=50844；705×52=36660；840×90=75600；690×98=67620505×60=30300；989×80=79120．
956+237=1193620-338=282505-238=267542-476=661000-678=322354+177=531
505×5 =2525 列竖式
1)13²=84+85 2)n²=[(n²+1)/2-1+(n²+1)/2]