Pembahasan:
Gunakan rumus berikut.
cosA – cosB = –2sin½(A + B)sin½(A – B)
sinA – sinB = 2cos½(A + B)sin½(A – B)
cos 140° – cos 100°
sin 140° – sin 100°
= –2sin½(140° + 100°)sin½(140° – 100°)
2cos½(140° + 100°)sin½(140° – 100°)
= –2sin½(240°)
2cos½(240°)
= –sin 120°
cos 120°
= √3/2
½
= _ √3 × (_ 2)
2 1
= √3