同理,』ydc=3nR I=crr+=rr +=r3 +=R3 3 3 =CnR+ 2TR K心, 3 2 , 3 ydzdx R S 同理  =  2 3 3 3 3 2 3 2 3 2  I = cR + R + R + R 2 . 2 3 = cR + R