\[\color {purple} {Example\ 1:}\ \color {red} {Find\ the\ value\ of}\ cos\ {40}^0\ cos\ {20}^0\ -\ sin\ {40}^0\ sin\ {20}^0\ \hspace{15cm}\]
\[\color {blue}{Solution:}\ W.\ K.\ T\ Cos A\ Cos B\ -\ Sin A Sin B\ =\ Cos( A + B )\ \hspace{18cm}\]
\[=\ Cos\ {60}^0\ \hspace{10cm}\]
\[=\ \frac{1}{2}\ \hspace{10cm}\]
\[\boxed{\therefore\ cos\ {40}^0\ cos\ {20}^0\ -\ sin\ {40}^0\ sin\ {20}^0\ =\ \frac{1}{2}}\]