a)
[tex]a_3=\frac{3-1}{3^2}=\frac{2}{9}\\a_7=\frac{7-1}{7^2}=\frac{6}{49}\\a_{28}=\frac{28-1}{28^2}=\frac{27}{784}[/tex]
b)
[tex]a_3=(-1)^3*2*(3^2-3)=-2*(9-3)=-2*6=-12\\a_7=(-1)^7*2*(7^2-3)=-2*(49-3)=-2*46=-92\\a_{28}=(-1)^{28}*2*(28^2-3)=2*(784-3)=2*781=1562[/tex]
c)
[tex]a_3=\sqrt{(3-1)(3+1)}+3*3=\sqrt{2*4}+9=2\sqrt2+9\\a_7=\sqrt{(7-1)(7+1)}+3*7=\sqrt{6*8}+21=\sqrt{48}+21=\sqrt{16*3}+21=4\sqrt3+21\\a_{28}=\sqrt{(28-1)(28+1)}+3*28=\sqrt{27*29}+84=\sqrt{783}+84=\sqrt{9*87}+84=3\sqrt{87}+84[/tex]
