Question

Prove that cos 20 + cos 100 +cos 140 = 0

Answer 1

Answer:

Step-by-step explanation:

cos20∘+cos100∘=cos(60∘−40∘)+cos(60∘+40∘)=cos40∘

by using the addition formulae for cos.

Then cos140∘=cos(180∘−40∘)=−cos(40∘). So

cos20∘+cos100∘+cos140∘=cos40∘−cos40∘=0

Answer 2

by using two times cos(C)+cos(d)