Two vectors of equal magnitudes having a sum of resultant equal to either of them, than the angel between them will be

Options

A. `50^(@)`
B. `60^(@)`
C. `75^(@)`
D. `120^(@)`

Answer

D

Explanation

`R^(2) =P^(2)+Q^(2) +2PQ cos theta`
It is given that R = P = Q
`therefore P^(2) = P^(2) +P^(2) +2P^(2) cos theta`
`therefore 2P^(2) cos theta =- P^(2)`
`therefore 1 =2 (1 + cos theta)" " therefore 1 + cos theta =(1)/(2)`
`cos theta =-(1)/(2)" "therefore theta =120^(@)`