TDSM 8.3
From The Data Science Design Manual Wikia
Assume that [math]A[/math] is a [math]m \times n[/math] matrix, [math]B[/math] is a [math]n \times p[/math] matrix, [math]C[/math] is a [math]p \times q[/math] matrix.
[math]\Rightarrow (AB)C = \sum_{k=1}^{p}\sum_{j=1}^n (a_{ij}b_{jk})c_{kl}[/math] = [math]\sum_{j=1}^n\sum_{k=1}^{p} a_{ij}(b_{jk}c_{kl})[/math] (number multiplication is associative) [math] = A(BC)[/math] (proved)
