a,i-r62 ce"rb2 (c2-b)r ao r0 210 2bcr a=VF-roy+(ro+2ror =(+c)r 。2 14 a r r c e rb c b r c t r ( ) 2 2 2 2 2 = −  = − = −   a = r + 2r = 2bcr    a (r r ) (r 2 r ) (b c )r 2 2 2 2 2 = − + + = + •• • • •    14