题目：求一个3*3矩阵对角线元素之和 1.程序分析：利用双重for循环控制输入二维数组，再将a[i][i]累加后输出。 2.程序源代码：-Topic: for a 3* 3 diagonal matrix elements combined 1. The program analysis: use double for cycle control input a 2 d array, again will be a [I] [I] after accumulation output