Question

Prove the following : cos 20° + cos 100° + cos 140° = 0

Answer 1

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

=2cos(60)cos(40)

=2*1/2*cos(40)

=cos 40

cos 20 + cos 100+ cos 140= cos 40+cos 140

= 2cos(40+140)/2cos(40-140)/2

=2cos 90 cos50

=2*0*cos50

=0

hope you understand