每个侧面积=3根5/4=0.75根5;设侧面斜高为h,(1+2)*h/2=0.75根5;h=0.5根5;
垂直高=根【(0.5根5)²-0.5²】=1;V=1/3*【1+4+根(1*4)】=7/3=2.33cm³.