Question

cos(20°) + cos(60°) + cos(100°) + cos(140°) =
1/2​

Answer 1

Answer:

Hope these answer may help you...

Answer 2

Step-by-step explanation:

cosA+cosB= 2cos((A+B)/2)cos((A-B)/2)

cos(180-A)= -cosA [A=any angel]

cos20+cos100

= 2cos((20+100)/2)cos((100-20)/2)

=2cos60 cos40 [cos60=1/2]

=cos40

cos20+cos60+cos100+cos140

=cos40+cos60+cos140

=cos40+1/2+cos(180-40)

=cos40+1/2-cos40

=1/2 [proved]