Gọi M = cos^4 15 độ − sin^4 15 độ thì:

Chọn B

\(\begin{array}{l}M = {\cos ^4}{15^{\rm{o}}} - {\sin ^4}{15^{\rm{o}}} = {\left( {{{\cos }^2}{{15}^{\rm{o}}}} \right)^2} - {\left( {{{\sin }^2}{{15}^{\rm{o}}}} \right)^2} = \left( {{{\cos }^2}{{15}^{\rm{o}}} - {{\sin }^2}{{15}^{\rm{o}}}} \right)\left( {{{\cos }^2}{{15}^{\rm{o}}} + {{\sin }^2}{{15}^{\rm{o}}}} \right)\\\,\,\,\,\,\,\, = {\cos ^2}{15^{\rm{o}}} - {\sin ^2}{15^{\rm{o}}} = \cos \left( {{{2.15}^{\rm{o}}}} \right) = \cos {30^{\rm{o}}} = \frac{{\sqrt 3 }}{2}.\end{array}\)