$\cos \left( {\left( P \right),\left( Q \right)} \right) = \dfrac{{\left| {a.a' + b.b' + c.c'} \right|}}{{\sqrt {{a^2} + {b^2} + {c^2}} .\sqrt {a{'^2} + b{'^2} + c{'^2}} }}$          

$\cos \left( {\left( P \right),\left( Q \right)} \right) = \dfrac{{a.a' + b.b' + c.c'}}{{\sqrt {{a^2} + {b^2} + {c^2}} .\sqrt {a{'^2} + b{'^2} + c{'^2}} }}$

$\cos \left( {\left( P \right),\left( Q \right)} \right) = \dfrac{{a.a' + b.b' + c.c'}}{{\sqrt {a + b + c} .\sqrt {a' + b' + c'} }}$      

$\cos \left( {\left( P \right),\left( Q \right)} \right) = \dfrac{{\left| {a.a' + b.b' + c.c'} \right|}}{{{{\sqrt {a + b + c} }^2}.{{\sqrt {a' + b' + c'} }^2}}}$