Tìm hệ số \({x^2}\) trong khai triển \({\left( {1 - 2x} \right)^8}\).

Gọi AH là đường phân giác góc A.

Áp dụng tính chất đường phân giác cho ∆ABC: \(\frac{{AB}}{{AC}} = \frac{{BH}}{{HC}}\)

\(\frac{{AB}}{{BH}} = \frac{{AC}}{{HC}} = \frac{{AB + AC}}{{BH + HC}} = \frac{{2 + 3}}{{BC}} = \frac{5}{{\sqrt 7 }}\)

\( \Rightarrow BH = AB:\frac{5}{{\sqrt 7 }} = \frac{{2\sqrt 7 }}{5}\)

\(\cos \widehat B = \frac{{A{C^2} - A{B^2} - B{C^2}}}{{ - 2AB.BC}} = \frac{{\sqrt 7 }}{{14}}\)

Xét ∆ABH có: \(A{H^2} = A{B^2} + B{H^2} - 2.AB.BH.cos\widehat B = \frac{{108}}{{25}} \Rightarrow AH = \frac{{6\sqrt 3 }}{5}\).

Ta có: 0 < x < \(\frac{\pi }{2} \Rightarrow \left\{ {\begin{array}{*{20}{c}}{\sin x > 0}\\{\cos x > 0}\end{array}} \right.\)

+) \({\cos ^2}x + {\sin ^2}x = 1 \Leftrightarrow {\frac{2}{{\sqrt 5 }}^2} + {\sin ^2}x = 1\)

\( \Leftrightarrow {\sin ^2}x = \frac{1}{5} \Leftrightarrow \left[ {\begin{array}{*{20}{c}}{\sin x = \frac{1}{{\sqrt 5 }}\left( {TM} \right)}\\{\sin x = - \frac{1}{{\sqrt 5 }}\left( L \right)}\end{array}} \right.\)

\( + )1 + {\cos ^2}x = \frac{1}{{{{\cos }^2}x}} \Leftrightarrow 1 + {\cos ^2}x = \frac{1}{{{{\left( {\frac{2}{{\sqrt 5 }}} \right)}^2}}}\)

\( \Leftrightarrow {\tan ^2}x = \frac{1}{4} \Leftrightarrow \left[ {\begin{array}{*{20}{c}}{\tan x = \frac{1}{2}(TM)}\\{{\mathop{\rm t}\nolimits} = - \frac{1}{2}(L)}\end{array}} \right.\).