Question

(xi)
cos 80° + cos 40º - cos 20º = 0​

Answer 1

Answer:

cos80°+cos40°-cos20°=0

we know that;

[cosx+cosy]=2cos(x+y)/2.cos(x-y) /2]

so,

cos80°+cos40°=2cos(80°+40°) /2.cos(80°-40°) /2

= 2cos60°(cos20°) -cos20°

=cos20°(2.1/2-1)

=cos20°(1-1)

=cos20°(0)

=0

I hope it will help u