hứa chọn ctlhn ạaaaaaaaa

$\begin{array}{l} CMR:\cot a - \tan a = 2\cot 2a\\ \cot a - \tan a = \dfrac{{\cos a}}{{\sin a}} - \dfrac{{\sin a}}{{\cos a}} = \dfrac{{{{\cos }^2}a - {{\sin }^2}a}}{{\sin a.\cos a}} = \dfrac{{\cos 2a}}{{\dfrac{1}{2}\sin 2a}} = 2\cot 2a\\ \cot 7,{5^o} + \tan 67,{5^o} - \tan 7,{5^o} - \cot 67,{5^o}\\  = \left( {\cot 7,{5^o} - \tan 7,{5^o}} \right) - \left( {\cot 67,{5^o} - \tan 67,{5^o}} \right)\\  = 2\cot {15^o} - 2\cot {135^o} = 6 + 2\sqrt 3 \\  \Rightarrow a = 6,b = 2\\  \Rightarrow 2a - 3{b^2} + ab = 2.6 - 3.4 + 6.2 = 12 \end{array}$