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Câu 1: B
$\displaystyle  \begin{array}{{>{\displaystyle}l}}
M\ =\ \cos^{4} 15^{0} \ -\ \sin^{4} 15^{0}\\
=\ \left(\cos^{2} 15^{0} \ +\ \sin^{2} 15^{0}\right)\left(\cos^{2} 15^{0} \ -\ \sin^{2} 15^{0}\right)\\
=\ \cos^{2} 15^{0} \ -\ \sin^{2} 15^{0}\\
=\ \cos 30^{0} \ =\ \frac{\sqrt{3}}{2}
\end{array}$