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VT $=\cos 6^{\circ}\cdot\cos 42^{\circ}\cdot\cos 66^{\circ}\cdot\cos 78^{\circ}$
$=\frac{1}{4\cos \left(60^{\circ}-6^{\circ}\right)}4\cos\left(60^{\circ}-6^{\circ}\right)\cdot\cos 6^{\circ}\cdot\cos\left(60^{\circ}+6^{\circ}\right)\cdot\cos 42^{\circ}\cdot\cos 78^{\circ}$
$=\frac{1}{4\cos 54^{\circ}}\cos3\left(6^{\circ}\right)\cdot\cos 42^{\circ}\cdot\cos 78^{\circ}$
$=\frac{1}{16\cos 54^{\circ}}4\cos \left(60^{\circ}-18^{\circ}\right)\cdot\cos 18^{\circ}\cdot\cos \left(60^{\circ}+18^{\circ}\right)$
$=\frac{1}{16\cos 54^{\circ}}\cos 3\left(18^{\circ}\right)$
$=\frac{1}{16}$
Trong đó đã sử dụng bài toán phụ
$4\cos\left(60^{\circ}-A\right)\cdot\cos A\cdot\cos\left(60^{\circ}+A\right)=\cos 3A$
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