Which of the following expressions could not be used to determine the exact value of cos 120°?

cos2(60°) − sin2(60°)
2cos2(60°) − 1
2sin(60°)cos(60°)
1 − 2sin2(60°)

Question

Which of the following expressions could not be used to determine the exact value of cos 120°?

cos2(60°) − sin2(60°)
 2cos2(60°) − 1
 2sin(60°)cos(60°)
 1 − 2sin2(60°)

jhonyy9 · Answer

try it in this way

120 = 60 + 60

cos 120° = cos (60+60) = ?

use formula cos (a+b)= ?

kekeman · Answer

@jhonyy9 wrote:

try it in this way 120 = 60 + 60 cos 120° = cos (60+60) = ? use formula cos (a+b)= ?

I think the answer is 2sin(60°)cos(60°)