5(b)解: 12X的./h)2 hh 3 hh 2 hb blh bh Ⅰ=0+bh 2八25 12 2 3 3 0 2 2 4 3 2 3 3 2 2 (b). I bh bh h bh I hb I bh b h b h y z yz 解： = +        = = = +  −             = −