Đề bài

Chứng minh đẳng thức sau \({\sin ^4}a + {\cos ^4}a = 1 - \frac{1}{2}{\sin ^2}2a = \frac{3}{4} + \frac{1}{4}\cos 4a\).

Lời giải chi tiết

\(\begin{array}{l}{\sin ^4}a + {\cos ^4}a = {\sin ^4}a + 2{\sin ^2}a{\cos ^2}a + {\cos ^4}a - 2{\sin ^2}a{\cos ^2}a\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = ({\sin ^4}a + 2{\sin ^2}a{\cos ^2}a + {\cos ^4}a) - \frac{1}{2}.4{\sin ^2}a{\cos ^2}a\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = {\left( {{{\sin }^2}a + {{\cos }^2}a} \right)^2} - \frac{1}{2}{(2{\mathop{\rm sinacosa}\nolimits} )^2}\,\,\,\,\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 1 - \frac{1}{2}{\sin ^2}2a\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 1 - \frac{1}{2}\left( {\frac{{1 - \cos 4a}}{2}} \right) = 1 - \frac{{1 - \cos 4a}}{4}\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \frac{3}{4} + \frac{1}{4}\cos 4a.\end{array}\)