2.52*【5又1/2-1.75*（2/3+9/21）】

= 2.52*{11/2 - 175*(14/21 + 9/21)/100}
= 2.52*{11/2 - 7*25*23/2100}
= 2.52*{11*150/300 - 25*23/300}
= 2.52*(11*6 - 23)*25/300
= 2.52*43*25/300
= 252*43*25/30000
= 84*43*25/10000
= 21*43*4*25/10000
= 21*43/100
= [20*43 + 43]/100
= [860 + 43]/100
= 903/100
= 9.03