Consider the binormal vectorB(t)=T(t)xN(t). Show that its derivativeB'(t) is a scalar multiple ofN(t).

So far I can prove that:

B'=T'xN+TxN'

B'=T'x(T'/|T'|)+TxN'

B'=0+TxN'

B'=TxN'

That doesn't prove the problem, any ideas?

Thanks!