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Tips on buying outdoor solar lights(太阳能灯)As solar lighting is becoming useful for homes, many different such outdoor lights can now be found in supermarkets. Here are some helpful tips for customers.Does your area receive enough direct lighting?【1】 So you shouldn’t even give it a thought if you live in an area that doesn’t get much sunlight. These lights need direct sunlight, or they just won’t work very well.【2】The expensive one usually means good quality. Choosing the cheapest ones will result in many problems. For once, it’s best to buy more expensive lights of high （2003•宁夏）已知⊙O的半径为r，那么，该圆两条平行切线间的距离为    ．