torsion formula for a parametric space curve I managed to prove 
$$ k(t) = \frac{ | a^{\prime} \times a^{\prime \prime} | }{ | a^{\prime} | ^3 } $$
for a regular parametric curve $a : I \to R^3 $ where $k(t)$ stands for its curvature but I am stucked in proving 
$$ \tau (t) = \frac{( a^{\prime} \times a^{\prime \prime} ) \cdot a^{\prime \prime \prime }}{ |a^{\prime} \times a^{\prime \prime} | ^2 } $$
Can I ask anybody to give me any hint or reference?
Thanks in advance.

It's going to take too long to type it all out in MathJax, so I have uploaded two photos of the derivations of the formulas for curvature and torsion.enter image description here


