Find the acute angle between the following planes: (i) $\overrightarrow r .(2\widehat i - 3\widehat j + 4\widehat k) = 1$ and $\overrightarrow r .(- \widehat i - \widehat j) = 4$ (ii) $\overrightarrow r .(2\widehat i - 3\widehat j + 6\widehat k) = 8$ and $\overrightarrow r .(3\widehat i + 4\widehat j - 12\widehat k) + 7 = 0$.