25/39÷(10/13+7/26)×1/2=25/39÷27/26×1/2=25/39×26/27×1/2=25/81
这个是25/39÷(10/13+7/26)x1/225/39x1/2÷(20/26+7/26)25/78÷27/2625/78x26/2726/78x25/271/3x25/2725/81
25/39÷(10/13+7/26)*1/2=25/39÷27/26*1/2=25/39*26/27*1/2=25*2/(3*27)*1/2=25/81