pt <=> $cos \frac{\pi }{6}.sin2x - sin\frac{\pi }{6}.cos2x=-cosx$ <=> $sin(2x-\frac{\pi }{6})=sin(\frac{\pi }{2}-x)$==> Tự Giải
pt <=> $cos \frac{\pi }{6}.sin2x - sin\frac{\pi }{6}.cos2x=-cosx$ <=> $sin(\frac{\pi }{6}-2x)=sin(\frac{\pi }{2}-x)$==> Tự Giải
pt <=> $cos \frac{\pi }{6}.sin2x - sin\frac{\pi }{6}.cos2x=-cosx$ <=> $sin(
2x-\frac{\pi }{6})=sin(\frac{\pi }{2}-x)$==> Tự Giải