題組內容

二、(50%)(所有題目請敘述計算過程，無計算過程不給分)
1. (10%) Let $A, B, C$ be $p \times q, m \times n, n \times p$ matrices and denote the $(i, j)$-entries of $A, B, C$ by $a_{ij}, b_{ij}, c_{ij}$ respectively.

(a) (5%) Please write down the \( (i,j) \)-entry of the product \( BCA \) in terms of Sigma notations (i.e. \( \Sigma \)).