S gia c = Ph l L trng c = Ltri + L phi B mt = S khun mi/ H kt cu o nn = o nn + o ta luy tri + o ta luy phi + o rnh tri + o rnh phi p nn = p nn + o khun mi + o gia c Vt bn (Vt hu c) ? - B vnh khun mi S gia c S khun mi o rnh = o rnh tri + o rnh phi o khun = o khun mi + o gia c Vt bn = Vt hu c = nh cp = B vnh = B vnh khun mi