sin 140 -sin 100 +sin 20

sin140-sin100=2cos1/2(140+100).sin1/2(140-100)
2cos(120).sin(20)=2-.1/2.sin(20)
=-sin(20)=sin(-20)
sin(-20)+sin(20) = 2sin1/2(-20+20).cos1/2(-20-20) = 2sin(0).cos(-20)= 0
sin0=0