一系统由A、B、C三个子系统串联而成，它们的寿命均服从指数分布，其中A、B的MTBF分别是200h、400h。现要求系统的MTBF在100h以上，则子系统C的MTBF至少为(3)h。

(74)

(75)

(73)

(72)

(70)

Although　the　bulk　of　industry　resources　and　energies　have　focused　on　developing　the　fastest(71)or　slickest(72),　more　and　more　mindshare　is　turning　to　the　evolution　of　the　computer　interface.　Advancements　in　the　areas　of　input　devices,(73)processing　and　vitual　reality　could　lead　to　fundamental　changes　in　the　way　human　and　computer　interact.　The　technological　battlefield　of　the　future　will　be　adding　layers　between　the　user　and　the　raw　machine　to　make　the(74)as　invisible　as　possible.(75)crepresents　the　next　evolutionary　step　for　the　interface.

(69)

(67)

(68)

(65)