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If vec a+ vec b+ vec c= vec0, show that...

If ` vec a+ vec b+ vec c= vec0,` show that the angle `theta` between the vectors ` vec b\ a n d\ vec c` ig givne by `costheta=(| vec a|^2-| vec b|^2-| vec c|^2)/(2| vec b|| vec c|)dot`

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Given that,
` vec a+ vec b+ vec c= vec0,`
`=> vec b+ vec c =-vec a`
`=>(vec b+ vec c)(vec b+ vec c) =(-vec a)(- vec a)`
`=>|vec b|^2+|vec c|^2+2 |vec b||vec c| cos theta = | vec a|^2`
`=>cos theta =(| vec a|^2-|vec b|^2-|vec c|^2)/(2 |vec b||vec c|`
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