7. (20%) Given $\varphi_1(t) = 1$, $\varphi_2(t) = t$, $\varphi_3(t) = 3t^2 - 1$,

i) Show that \(\varphi_1(t)\) and \(\varphi_2(t)\) are mutually orthogonal over the interval (-2, 2)