Cho cosa=-4/5 (π/2
Cho cosa=-4/5 (π/2
By Vivian
By Vivian
By Vivian
Cho cosa=-4/5 (π/2
$\cos2a=2\cos^2a-1=\dfrac{7}{25}$
$\dfrac{\pi}{2}<a<\pi$
$\Rightarrow \sin a>0, \cos a<0$
$\Rightarrow \sin a=\sqrt{1-\cos^2a}=\dfrac{3}{5}$
$\sin2a=2\sin a\cos a=\dfrac{-24}{25}$
$\to \sin2a.\cos2a=\dfrac{-168}{625}$
Đáp án:
$π/2<a<π⇔sina>0$
Có $cos²a+sin²a=1⇒sin²a=1-16/25=9/25⇔sina=3/5$
$cos2a.sin2a=(1-2sin²a).2cosasina$
$=(1-2.9/25).2.(-4/5).3/5$
$ =-168/625$
#NOCOPY