若使Z=2,必有 R R R R32+X R+X R 或 R+ x R+X X R 且 于是可以得到: (l+Q2)R、当Q210时 Rp=eRS (1+x2)X 12.3且 S P S P X R Q R X = = 于是可以得到： 2 2 (1 ) 1 (1 ) p e S p S e R Q R X X Q  = +   = +   当 10 Qe  时 2 p e S p S R Q R X X  =   =  1.2.3 若使 Z Z p S = ，必有 2 2 2 2 2 2 p S p p p p S p p p X R R R X R X X R X   = +    =  +  或 2 2 2 2 2 S S p S S S p S R X R R R X X X  +  =    + =  