=21(48+13×4)=21×(48+52)=21×100=2100
解:原式=21x4x12+84x13 =21x4x12+21x4x13 =21x4x(12+13) =21x(4x25) =21x100 =2100
=21x48+21x4x13=21x48+21x52=21x(48+52)=21x100=2100