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Given four non zero vectors a ,b ,c and d. The vectors c,b,a are coplanar but not collinear pair by pair and vector d is not coplanar with vectors a ,b,c andangle between ab is $\pi /3$ and between bc is $\pi /3$ and angle between a,d is m and between b,c is n then we have to prove that angle between c and d is $\cos^{-1} (\cos n- \cos m$ )

I am not getting any start how to do it , can anybody help me in this

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    This can't be right. I suspect that you made a typo – 2017-01-21
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    I suppose that $n$ is the angle between $c$ and $d$. – 2017-01-21
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    @polfosol this is the original question http://i.imgur.com/XMvjH8C.jpg – 2017-01-21
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    @zoli no it is the angle between b and d . And we have to find out angle between c and d – 2017-01-21
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    @koolman Image was not found (Error 404) – 2017-01-21
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    @polfosol that is working – 2017-01-21
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    https://s23.postimg.org/ha4g37u17/Screenshot_2017_01_21_16_52_38_1.jpg – 2017-01-21
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    Than no need for $-1$ in exponentation in second and third cosines. – 2017-01-21
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    @kolobokish oh sorry – 2017-01-21
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    See what means that $ab$ and $bc$ form angles with magnitude $\frac{\pi}{3}$. $ca$ also have angle $\frac{\pi}{3}$. (equilateral triangle.) – 2017-01-21
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    @kolobokish then what to do next – 2017-01-21

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