皮皮学，免费搜题
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【简答题】
如图， l 1 ， l 2 分别表示一种白炽灯和一种节能灯的费用y元（费用灯的售价+电费）与照明时间x小时的函数图象，假设两种灯的使用寿命都是2000小时，照明效果一样。 （1）根据图象求 l 1 ， l 2 的函数表达式； （2）当照明时间为多少时，两种费用相等？ （3）房间计划照明2500小时，他买了一个白炽灯和一个节能灯，请你帮助设计一种最省钱的方案（直接给出答安，不必写过程）
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皮皮学刷刷变学霸
举一反三
【判断题】Two lines are parallel in space provided that their respective projections on two projection planes are parallel.
A.
正确
B.
错误
【判断题】If two lines in space are intersection, their projections will intersect also.
A.
正确
B.
错误
【简答题】n. 指示,表示,表明
【判断题】Whe two neighboring surfaces are in tangent condition, there is no line between their projections
A.
正确
B.
错误
【简答题】指示,表明,表示 n.
【单选题】What two types of projections give a pictorial view of the object without convergence?
A.
orthographic and perspective
B.
oblique and axonometric
C.
perspective and oblique
D.
isometric and orthographic
【单选题】Which of the following statements about projections is NOT true?
A.
The center of parallel projection is placed at a finite distance from the view plane.
B.
Projectors are not parallel to each other in perspective projections.
C.
In perspective projections, objects away from the viewpoint or center of projection appear smaller comparing to identical objects closer to the center of projection
D.
Proportions are maintained in parallel projections.
【简答题】指示;表明;表示
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