Giải Thực hành 3 trang 22 Toán 11 tập 1 Chân trời

$sin\frac{\pi }{24}cos\frac{5\pi }{24} = \frac{1}{2}\left [ sin(\frac{\pi }{24}-\frac{5\pi }{24})+sin(\frac{\pi }{24}+\frac{5\pi }{24} \right ] = \frac{1}{2}\left [ sin(\frac{-\pi }{6})+sin(\frac{\pi }{4})\right ]= \frac{1}{2}.(\frac{-1}{2}+\frac{\sqrt{2}}{2}) = \frac{\sqrt{2}-1}{4}$

$sin\frac{7\pi }{8}sin\frac{5\pi }{8} = \frac{1}{2}\left [ cos(\frac{7\pi }{8}-\frac{5\pi }{8})-cos(\frac{7\pi }{8}+\frac{5\pi }{8}) \right ] = \frac{1}{2}\left [ cos(\frac{\pi }{4})-cos(\frac{3\pi }{2})\right ]= \frac{1}{2}.(\frac{\sqrt{2}}{2}-0) = \frac{\sqrt{2}}{4}$