The involute function Inv $\left ( \varphi \right )$ and the pressure angle $\varphi$ are correlated by which one of the following expressions?
(a) Inv ($\varphi$) = tan($\varphi$-1) (b) Inv ($\varphi$) = tan$\varphi$+$\varphi$
(c) Inv ($\varphi$) = 1-tan $\varphi$ (d) Inv ($\varphi$) = tan$\varphi$-$\varphi$