Acute angle between vector (Proof)

Question

\(Sin^2 = (a × b)^2 ÷ /a/^2 /b/^2\)

Show that \(Thetha\) is the acute angle between the vector \(a\) and \(b\) hence or otherwise find the sine of the angle between the vectors, 

\(a= 3i + j + 2k\)
\(b = 2i - 2j + 4k\)