VectorEasy
Question
Two equal vectors have a resultant equal to either of them, then the angle between them will be:
Options
A.110o
B.120o
C.60o
D.150o
Solution
Applying the formula,
R2 = P2 + Q2 + 2PQ cos α
P2 = P2 + P2 + 2PP cos α
= 2P2 + 2P2 cosα = 2P2 (1 + cosα)
1 + cos α =
⇒ 2 cos2α 
cos2 α
⇒ cos 
= cos 60o
= 60o ⇒ α = 120o
R2 = P2 + Q2 + 2PQ cos α
P2 = P2 + P2 + 2PP cos α
= 2P2 + 2P2 cosα = 2P2 (1 + cosα)
1 + cos α =
⇒ 2 cos2α cos2 α
⇒ cos = cos 60o = 60o ⇒ α = 120oCreate a free account to view solution
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