题目

 Students in American schools learn from an early age to give presentations(演讲) as part of their regular classroom activities. Children as young as five years old often give brief talks about objects they bring in to school – called “show and tell,” and this training is a basis for later public speaking. Even so, many native English-speaking adults are afraid to speak or give presentations in front of a large group. Speaking English in public meetings is necessary for many students and employees. The best way to improve is to practice public speaking in a friendly environmen4．已知长方体AC1中，棱AB=BC=1，棱BB1=2，点E为棱BB1上的点．（Ⅰ）求证：AC⊥BD1；（Ⅱ）求证：平面D1DB⊥平面ACE；（Ⅲ）BE=$\frac{1}{4}$BB1，求平面ACE与平面ACD1所成角的余弦值．