如图所示，当双刀双掷电键悬空时，若用一平行单色光照射光电管阴极K，发生了光电效应，现将双刀双掷电键拨向bb′，用同样的光照射光电管，并使变阻器的片P自左向右移动，当电压表示数为3.1V时，电路中的电流恰为零，若将电键拨向aa′并移动变阻器的滑片P，使电压表示数变为4.5V，此时电子到达A极的最大动能为____eV．

长江和黄河干流共同发源于（　　）

    In May, 1976, Gaynor was a nine-year-old girl and lived in North Wales. One day, on her way home from school, she saw a silver(银色的)object(物体)in a field not far from her home. The object looked like a saucer.
    Gaynor was very scared. She stayed behind the tall grass and watched the door of the strange object open. She saw two aliens wearing silver suits get out. Then they used something to examine(检测)the ground. They were short and had pink eyes. After about thirty minutes, the two aliens went back into the object. Then it took off.
    When Gaynor got home, she told her mother about this unusual experience, but her family decided to keep quiet about it. They didn’t want others to laugh at their daughter.
    However, 18 months later, Gaynor told her story to UFO scientists. They found out that Gaynor told the truth.

(12分) 阅读并解答问题
用配方法可以解一元二次方程，还可以用它来解决很多问题．例如：因为，所以就有最小值1，即，只有当时，才能得到这个式子的最小值1．同样，因为，所以有最大值1，即，只有在时，才能得到这个式子的最大值1．

(1)当=      时，代数式有最      （填写大或小）值为          ．
(2)当=      时，代数式有最      （填写大或小）值为         ．
(3)矩形花园的一面靠墙，另外三面的栅栏所围成的总长度是16m，当花园与墙相邻的边长为多少时，花园的面积最大？最大面积是多少?

甲烷广泛存在于天然气、沼气、煤矿坑气之中，是优质的气体燃料，更是制造许多化工产品的重要原料．