[tex]\it \dfrac{7}{10} = \dfrac{1}{5} + \dfrac{1}{2} \\ \\ \\ \dfrac{8}{15} = \dfrac{1}{5} + \dfrac{1}{3}\\ \\ \\ \dfrac{9}{20} = \dfrac{1}{5} + \dfrac{1}{4}\\ \\.\\.\\.\\ \\ \dfrac{85}{400} = \dfrac{1}{5} + \dfrac{1}{80} \\ \\ \\ a=\dfrac{1}{5}\cdot80 + \Big(\dfrac{1}{2}+\dfrac{1}{3}+\dfrac{1}{4}+\ ...\ +\dfrac{1}{80} \Big) - \Big(\dfrac{1}{2}+\dfrac{1}{3}+\dfrac{1}{4}+\ ...\ +\dfrac{1}{80} \Big) =16=4^2[/tex]
