简述正投影法的基本特性。
参考答案：
（1）真实性.当直线、曲线成平面平行于投影面时，直线或曲线的投影反映实长，平面的投影反映真实形状。
（2）积聚性。当直线、曲线或平面垂直于投影面时，直线的投影枳聚成一点，平面或曲线的投影积聚成直线成曲线。
（3）类似性。当直线、曲线或平面倾斜于投影面时，直线或曲线的投影仍然为直线或曲线、但小于实长。平面的投影小于实际形状的大小，是实际形状的类似形。
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| Mothers' Day is coming near.I am 1 what present I should give to my mother. She is a factory worker and 2 very hard. She has to do all the housework herself because my father is a pilot and he is 3 at home. My family is not rich but my mother tries her best to 4 my old grandparents. Also she 5 a lot of attention to my studies. She doesn't buy 6 new clothes for herself but she saved money to buy a computer for me.I love her very much. Just flowers or a card can not show my love to my mother and she will be unhappy with me if I spend money 7 such things. And I think my prize in the English competition (竞赛) must be the best 8 for her.I will show her my prize and say,"I am very 9 to you and you are the greatest mother in the world. " 10 I will give her a long,deep kiss. I'll tell her I will love her forever. | |||
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