A$={\displaystyle \underset{1}{\overset{b}{}}}\frac{1}{x^2}dx={\displaystyle \underset{1}{\overset{b}{}}}{x}^{-2}dx=[\frac{-1}{}]$ from $1$ to b$.$ i understand ${\displaystyle \underset{1}{\overset{b}{}}}{x}^{-2}$but how did we get $[\frac{-1}{x}$