If `veca and vecb` are two vectors, such that `veca.vecblt0 and |veca.vecb|=|vecaxxvecb|` then the angle between angles between the vectors `veca and vecb` is

Options

A. `pi`
B. `7pi//4`
C. `pi//4`
D. `3pi//4`

Answer

d

Explanation

`|veca.vecb|=|vecaxxvecb|`
`Rightarrow|veca||vecb||costheta|=|veca||vecb||sin theta|`
(where,`theta` is the angle between `veca and vecb`)
`Rightarrow |costheta|= |sintheta|`
`Rightarrow theta= pi/4 or (3pi)/4(as 0 le thetalepi)`
But `veca.vecb lt 0`, therefore, `theta= (3pi)/4`