11. 如右圖所示,邊長a = 400 mm,b = 300 mm,c = 200 mm之長方體,楊氏模數E = 200 GPa,蒲松比(Poisson’s ratio)為ν = 0.3,承受σx= -40 MPa,σy= -60 MPa以及σz= -20 MPa之三軸向應力,分別作用在x,y及z面上,則y軸邊長縮減量為何?