Use the power rule,
[tex]\displaystyle \int x^n \, dx = \frac{x^{n+1}}{n+1} + C[/tex]
with [tex]n=-8[/tex]. It follows by the fundamental theorem of calculus that
[tex]\displaystyle \int_1^7 \frac{dx}{x^8} = -\frac1{7x^7} \bigg|_{x=1}^{x=7} = -\frac17 \left(\frac1{7^7} - \frac1{1^7}\right) = \boxed{\frac17 - \frac1{7^8}}[/tex]
