[กิตติ]

ขอบคุณครับคุณPasser-by โดนดักอีกแล้ว แก้คำตอบแล้วครับ

28.$2\sin^260^\circ (\tan5^\circ +\tan85^\circ)-12\sin 70^\circ =?$

$\tan5^\circ +\tan85^\circ=\frac{\sin5^\circ}{\cos5^\circ} +\frac{\cos5^\circ}{\sin5^\circ} $
$=\frac{2}{\sin10^\circ} $

$2\sin^260^\circ (\tan5^\circ +\tan85^\circ)-12\sin 70^\circ $

$=2(\frac{\sqrt{3}}{2})^2(\frac{2}{\sin10^\circ})-12\cos 20^\circ$

$=\dfrac{3}{\sin10^\circ}-12\cos 20^\circ$

$=\dfrac{3-12\sin10^\circ\cos 20^\circ}{\sin10^\circ}$

$=\dfrac{3-6(\sin30^\circ-\sin 10^\circ)}{\sin10^\circ}$

$=6$