Đáp án:
\[A\]
Giải thích các bước giải:
\(\begin{array}{l}
A = {\sin ^2}\frac{\pi }{8} + {\sin ^2}\frac{{3\pi }}{8} + {\sin ^2}\frac{{5\pi }}{8} + {\sin ^2}\frac{{7\pi }}{8}\\
= {\sin ^2}\frac{\pi }{8} + {\cos ^2}\left( {\frac{\pi }{2} - \frac{{3\pi }}{8}} \right) + {\sin ^2}\left( {\pi - \frac{{5\pi }}{8}} \right) + {\cos ^2}\left( {\frac{\pi }{2} - \frac{{7\pi }}{8}} \right)\\
= {\sin ^2}\frac{\pi }{8} + {\cos ^2}\frac{\pi }{8} + {\sin ^2}\frac{{3\pi }}{8} + {\cos ^2}\left( { - \frac{{3\pi }}{8}} \right)\\
= {\sin ^2}\frac{\pi }{8} + {\cos ^2}\frac{\pi }{8} + {\sin ^2}\frac{{3\pi }}{8} + {\cos ^2}\frac{{3\pi }}{8}\\
= 1 + 1 = 2
\end{array}\)
