Write the Word or Phrase That Best Completes Each Statement $$1 \cdot 8 + 2 \cdot 8 + 3 \cdot 8 + \ldots + 8 n = \frac { 8 n ( n + 1 ) } { 2 }$$

Write the word or phrase that best completes each statement or answers the question. Use mathematical induction to prove that the statement is true for every positive integer n.
- $$1 \cdot 8 + 2 \cdot 8 + 3 \cdot 8 + \ldots + 8 n = \frac { 8 n ( n + 1 ) } { 2 }$$