17.2÷2+2.8×0.5 =17.2×0.5+2.8×0.5 =（17.2+2.8）×0.5 =20×0.5 =10 1.72÷4+2.28×0.25 =1.72×0.25+2.28×0.25 =（1.72+2.28）×0.25 =4×0.25 =1
