设直平行六面体为ABCD-A1B1C1D1,AC1和BD1为两条体对角线,AC1=15cm,BD1=20cm,△AC1C和△BDD1都是RT△,根据勾股定理,AC^2+CC1^2=AC1^2AC^2=15^2-12^2=81,AC=9cm,BD^2+DD1^2=BD1^2,BD^2=20^2-12^2=256,BD=16cm,在底面AB...