已知l是⊙O的切线，⊙O的直径AB=10cm，那么点A、B到直线l的距离之和为______cm．
根据题意画出图形，如图所示，设直线l与圆的切点为E，
过A作AC⊥l，垂足为C，过B作BD⊥l，垂足为D，连接OE，则OE⊥l，
当AB不平行l时，四边形ABDC为梯形，又O为AB的中点，∴OE为梯形的中位线，
∴OE=
（AC+BD），即AC+BD=2OE=AB=10cm，1 2
当AB∥l时，四边形ABDC为矩形，AC+BD=AB=10cm，
综上，点A、B到直线l的距离之和为10cm．
故答案为：10．
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| Have you ever planted a few trees on Tree Planting Day? Each year, millions of people, both 1 , plant a lot of trees. But this may not be the best way to make your home a 2 place. Last week we visited a park in Hebei. We were 3 to find most of the trees had been burned. The workers told us that the trees 4 soon after they were planted because they weren't watered well enough. 5 workers burned them and cleaned the place for 6 trees to be planted this year. 7 a card found in one of the dead trees, a student wrote:"I hope this tree will grow up with me to 8 the backbone (栋梁) of our country." The workers said that the survival (存活) of the trees was really more important than 9 trees were planted. Some people in Beijing now have a new idea that they can donate (损赠) some money and let professionals (专业人士) plant and 10 the trees. | ||||
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