Question

cos20°cos100°+cos100°cos140°-cos140°cos200°=-3/4
prove it its urgent!!​

Answer 1

Answer:

cos u x cos v = 1/2 x(cos (u-v) + cos(u+v))

1/2 x(cos (20–100) + cos (20+100)) + 1/2 x(cos (100–140) + cos (100+140)) - 1/2 x(cos (140–200) + cos (140+200)) = ?

1/2 x(cos (-80) + cos (120)) + 1/2 x(cos (-40) + cos (240)) - 1/2 x(cos (-60) + cos (340)) = ?

1/2 x(-.3263518223) + 1/2 x(.2660444431) - 1/2 x(1.43962621) = ?

-.1631759112 + (.1330222216) - .7198463104 = -.7501383206 approx. = -3/4