Cho hình thang \[ABCD\,\,\left( {AB\,{\rm{//}}\,CD} \right),\,\,\widehat C = 36^\circ \,;\,\,\widehat D = 50^\circ \]. Biết \(AB = 4\;{\rm{cm}},AD = 6\;{\rm{cm}}\). Tính chu vi hình thang.

Ta có \(\cos A = \frac{{AE}}{{AB}};\,\,\cos B.cosC = \frac{{BD}}{{AB}}.\frac{{DC}}{{AC}}\,\,\,\,\,\,\left( 1 \right)\)

Mặt khác

\( \Rightarrow \frac{{DB}}{{DA}} = \frac{{DH}}{{DC}} \Rightarrow DB.DC = DA.DH\)

Hay \(DB.DC = DA.AH\,\,\left( 2 \right)\)

Mà \( \Rightarrow \frac{{AE}}{{AD}} = \frac{{AH}}{{AC}} \Rightarrow AE.AC = AD.DH\,\,\left( 3 \right)\)

Từ \(\left( 1 \right),\,\left( 2 \right),\,\left( 3 \right)\) ta có \({\mathop{\rm cosB}\nolimits} .cosC = \frac{{AE.AC}}{{AB.AC}} = \frac{{AE}}{{AB}}\) suy ra \[\cos A = \cos B.cosC\]

a) Xét \[\Delta ABC\] vuông tại \[A\] có: O10-2024-GV154 \[AC = \sqrt {B{C^2} - B{A^2}}  = \sqrt {{{13}^2} - {5^2}}  = 12cm\]

Trong \[\Delta ABC\] vuông tại \[A\] có: O10-2024-GV154.\[\sin \widehat {ACB} = \frac{{AB}}{{BC}} = \frac{5}{{13}}\];\[{\rm{cos}}\widehat {ACB} = \frac{{AC}}{{BC}} = \frac{{12}}{{13}}\];

\[\tan \widehat {ACB} = \frac{{\sin \widehat {ACB}}}{{{\rm{cos}}\widehat {ACB}}} = \frac{5}{{12}}\];\[\cot \widehat {ACB} = \frac{1}{{{\rm{tan}}\widehat {ACB}}} = \frac{{12}}{5}\].

b)Áp dụng tính chất đường phân giác trong \[\Delta ABC\] ta có: O10-2024-GV154

\[\frac{{AB}}{{AE}} = \frac{{BC}}{{CE}} \Rightarrow \frac{{AE}}{5} = \frac{{CE}}{{13}} = \frac{{AE + CE}}{{5 + 13}} = \frac{{AC}}{{18}} = \frac{2}{3}\]

Vậy\[A{\rm{E}} = \frac{{10}}{3}cm.\]\[{\rm{CE}} = \frac{{26}}{3}cm.\]

\[\frac{{AC}}{{AF}} = \frac{{CB}}{{BF}} \Rightarrow \frac{{AF}}{{12}} = \frac{{BF}}{{13}} = \frac{{AF + BF}}{{12 + 13}} = \frac{{AC}}{{25}} = \frac{1}{5}\]

Vậy \[{\rm{AF}} = \frac{{12}}{5}cm.\]\[{\rm{BF}} = \frac{{13}}{5}cm.\]