Step-by-step explanation:
[tex] \Sigma \: (13 - 1) \\ = \Sigma \: 12 \\ = 1 + 2 + 3 + 4 + ..... + 12 \\ = \frac{12(12 + 1)}{2} \\ = \frac{12 \times 13}{2} \\ = 6 \times 13 \\ = 78 \\ \therefore \: \huge \orange{ \boxed{\Sigma \: (13 - 1) = 78}}[/tex]
