红线是夹角的另一边,因图没画好,怕重合故将上底面的线画到了下底面上.
(cosα)^2=红线1^2/对角线^2=(长^2+宽^2)对角线^2
(cosβ)^2=红线2^2/对角线^2=(长^2+高^2)对角线^2
(cosγ)^2=红线3^2/对角线^2=(高^2+宽^2)对角线^2
所以(cosα)^2+(cosβ)^2+(cosγ)^2=2(长^2+宽^2+宽^2)/对角线^2=2