设计一个界面 第一行有 按钮 第二行正中间 有 第三行当单击“=”按钮时，在第三个文本框中显示前两个加数的和-The first line of the design of an interface with buttons second row in the middle there is a third line of business, click " ="　button, the display in front of the third text box and two summand