例题1.展开下列求和式,(1)a1a;(2)a 解:(1)aa1=a1a1+a2a2+a13a3 (2) ajk l kaki t aiai arra =a1C1+a1221+a1331+a2112+a2a22+a23432 +a31a13+a32a23+a343例题1. 展开下列求和式， 解： 1 1 2 2 3 3 (1).ai ai = a a + a a + a a 3 1 1 3 3 2 2 3 3 3 3 3 1 1 1 1 1 2 2 1 1 3 3 1 2 1 1 2 2 2 2 2 2 3 3 2 1 1 2 2 3 3 (2). a a a a a a a a a a a a a a a a a a a a a a a a a a j k kj k k k k k k + + + = + + + + + = + + (1). ; (2). . ai ai a jkakj