Question

Please answer the given question.​

Answer 1

Step-by-step explanation:

$\sum ^{n} _{r = 1} (8r - 7)$

$= \sum ^{n} _{r = 1} 8r - \sum^{n}_{r = 1}7$

$=8 \frac{n(n + 1)}{2} - 7n$

$= 4n(n + 1) - 7n$

$= 4n^{2} + 4n- 7n$

$= 4n^{2} - 3n$

$= n(4n - 3)$