Giải phương trình \[\sqrt {\rm{3}} {\rm{tan2x}} - {\rm{3 = 0}}\]

\[{\mathop{\rm co}\nolimits} {\rm{s}}\left( {{\rm{4x}} - \frac{{\rm{\pi }}}{{\rm{6}}}} \right){\rm{ + si}}{{\rm{n}}^{\rm{2}}}{\rm{x = co}}{{\rm{s}}^{\rm{2}}}{\rm{x}} \Leftrightarrow {\mathop{\rm c}\nolimits} {\rm{os}}\left( {{\rm{4x}} - \frac{{\rm{\pi }}}{{\rm{6}}}} \right){\rm{ = co}}{{\rm{s}}^{\rm{2}}}{\rm{x}} - {\rm{si}}{{\rm{n}}^{\rm{2}}}{\rm{x}}\]

\[ \Leftrightarrow {\mathop{\rm c}\nolimits} {\rm{os}}\left( {{\rm{4x}} - \frac{{\rm{\pi }}}{{\rm{6}}}} \right){\rm{ = cos}}2x\]

\[ \Leftrightarrow \left[ {\begin{array}{*{20}{c}}{{\rm{4x}} - \frac{{\rm{\pi }}}{{\rm{6}}}{\rm{ = }}2x + k2\pi }\\{{\rm{4x}} - \frac{{\rm{\pi }}}{{\rm{6}}}{\rm{ = }} - 2x + k2\pi }\end{array}} \right. \Leftrightarrow \left[ {\begin{array}{*{20}{c}}{x\,{\rm{ = }}\,\,\frac{\pi }{{12}} + k\pi }\\{x\,{\rm{ = }}\,\,\frac{\pi }{{36}} + k\frac{\pi }{3}}\end{array}} \right.\left( {k \in \mathbb{Z}} \right)\]

Ta có mỗi họ nghiệm lần lượt có các nghiệm âm lớn nhất là: \[{{\rm{x}}_{\rm{1}}}{\rm{ = }}\frac{{\rm{\pi }}}{{{\rm{12}}}} - {\rm{\pi = }} - \frac{{{\rm{11\pi }}}}{{{\rm{12}}}}{\rm{;}}\,\,{{\rm{x}}_{\rm{2}}}{\rm{ = }}\frac{{\rm{\pi }}}{{{\rm{36}}}} - \frac{{\rm{\pi }}}{{\rm{3}}}{\rm{ = }} - \frac{{{\rm{11\pi }}}}{{{\rm{36}}}}\]

Vậy nghiệm âm lớn nhất của phương trình là \[{\rm{x = }} - \frac{{11}}{{36}}{\rm{\pi }}\]

Đáp án cần chọn là: B

\[{\rm{si}}{{\rm{n}}^{\rm{3}}}{\rm{x + sinxcosx = 1}} - {\rm{co}}{{\rm{s}}^{\rm{3}}}{\rm{x}} \Leftrightarrow {\rm{si}}{{\rm{n}}^{\rm{3}}}{\rm{x + co}}{{\rm{s}}^{\rm{3}}}{\rm{x = 1}} - {\rm{sinxcosx}}\]

\[ \Leftrightarrow \left( {{\rm{sinx + cosx}}} \right)\left( {{\rm{1}} - {\rm{sinxcosx}}} \right){\rm{ = 1}} - {\rm{sinxcosx}}\]

\( \Leftrightarrow \left[ {\begin{array}{*{20}{c}}{1 - sinxco{\rm{sx = 0}}}\\{{\rm{sinx + cosx = 1}}}\end{array}} \right. \Leftrightarrow \left[ {\begin{array}{*{20}{c}}{{\rm{sin2x = 2(VN)}}}\\{{\rm{sin}}\left( {{\rm{x + }}\frac{{\rm{\pi }}}{{\rm{4}}}} \right){\rm{ = }}\frac{{\rm{1}}}{{\sqrt {\rm{2}} }}}\end{array}} \right. \Leftrightarrow \left[ {\begin{array}{*{20}{c}}{{\rm{x = k2\pi }}}\\{{\rm{x = }}\frac{{\rm{\pi }}}{{\rm{2}}}{\rm{ + k2\pi }}}\end{array}} \right.,k \in \mathbb{Z}\)

Vì \[{\rm{x}} \in \left[ {{\rm{0; }}\frac{{\rm{\pi }}}{{\rm{2}}}} \right] \Rightarrow {\rm{x}} \in \left\{ {{\rm{0; }}\frac{{\rm{\pi }}}{{{\rm{ 2}}}}} \right\}\]Đáp án cần chọn là: C