(3/sin^2140°-1/cos^2140°)*1/2sin10°
=(3cos^2140-sin^2140)*0.5sin10/(sin^2140cos^2140)
=8(√3/2cos140-1/2sin140)(√3/2cos140+1/2sin140)/sin^2280
= -8sin80sin200/sin^2280
= -8sin80*sin160/sin^280
= -16sin^280*cos80/sin^280
= -16cos80