题目

如图，四棱锥的底面为矩形，且，，，. （1）求证：平面平面； （2）求三棱锥的体积. 答案：（1）证明：∵四边形为矩形， ∴⊥， 又∵∠， ∴⊥． 又∵， ∴⊥平面， ∵， ∴⊥平面， 又∵平面， ∴平面⊥平面． （2）解：， ∴．Best School BackpacksStudents of all ages need to have a reliable backpack and which is your favourite one?1. Amazon Basics Classic BackpackThe Amazon Basics line of products aims to deliver outstanding value by getting rid of everything that is unnecessary while keeping the most important function that customers desire. The Classic Backpack may not look like much, but that is a big part of its appeal. It’s cheap and cheerful, lightweight yet long-lasting with velcro（尼龙刺粘搭链）side pockets and a large center. With an average rating of 4.5 stars based on over 250 customer reviews, this is a well-made backpack at a terrific price.Price:From$18. 002. The North Face Refractor Duffel PackWant to head off to school with a slightly different backpack that isn’t way out there? The definitely boxy Refractor Duffel Pack from The North Face offers handles on both the top and the side, plus the usual shoulder belts, for several ways to carry. The clamshell design also includes a quick-access pocket on the front and an included tablet sleeve. l00% of reviewers on The North Face online store recommend this product, making it the best-selling unisex(男女通用的) backpack on the site. .Price: $155. 003. High Sierra Powerglide Rolling Laptop BackpackCarrying heavy textbooks and other supplies around all day can get awfully tiring on the shoulders, back and hips. An easier way to get between classes is with a rolling backpack like the High Sierra Powerglide. The multi-compartment design includes a cushioned sleeve for laptops with screens up to 17-inches in size, plus zippered accessory compartments for quick access. The strange and large bag is recommended by over 90% of the over 900 customers who reviewed it on eBags.Price: $ 60. 001.What is the most remarkable feature of the Classic Backpack?A. It has many functions. B. It doesn't have any spare parts.C. It is heavy but long-lasting. D. It only has a large center.2.What’s the convenience of the Refractor Duffel Pack?A. It is difficult to open and shut. B. It has extra pockets on the front.C. It offers different ways to carry. D. It can be carried from every side.3.Where is the High Sierra Powerglide different from the others?A. It has wheels. B. It is heavy to carry.C. It can contain textbooks. D. It has a place to use a computer.4.What do the three products have in common?A. They are all expensive. B. They are all sold only online.C. They all have large space inside. D. They are all given praise by customers.