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a)
Đặt \(A = \left( {2\sin {{30}^o} + \cos {{135}^o} - 3\tan {{150}^o}} \right).\left( {\cos {{180}^o} - \cot {{60}^o}} \right)\)
Ta có: \(\left\{ \begin{array}{l}\cos {135^o} = - \cos {45^o};\cos {180^o} = - \cos {0^o}\\\tan {150^o} = - \tan {30^o}\end{array} \right.\)
\( \Rightarrow A = \left( {2\sin {{30}^o} - \cos {{45}^o} + 3\tan {{30}^o}} \right).\left( { - \cos {0^o} - \cot {{60}^o}} \right)\)
Sử dụng bảng giá trị lượng giác của một số góc đặc biệt, ta có:
\(\left\{ \begin{array}{l}\sin {30^o} = \frac{1}{2};\tan {30^o} = \frac{{\sqrt 3 }}{3}\\\cos {45^o} = \frac{{\sqrt 2 }}{2};\cos {0^o} = 1;\cot {60^o} = \frac{{\sqrt 3 }}{3}\end{array} \right.\)
\( \Rightarrow A = \left( {2.\frac{1}{2} - \frac{{\sqrt 2 }}{2} + 3.\frac{{\sqrt 3 }}{3}} \right).\left( { - 1 - \frac{{\sqrt 3 }}{3}} \right)\)
\(\begin{array}{l} \Leftrightarrow A = - \left( {1 - \frac{{\sqrt 2 }}{2} + \sqrt 3 } \right).\left( {1 + \frac{{\sqrt 3 }}{3}} \right)\\ \Leftrightarrow A = - \frac{{2 - \sqrt 2 + 2\sqrt 3 }}{2}.\frac{{3 + \sqrt 3 }}{3}\\ \Leftrightarrow A = - \frac{{\left( {2 - \sqrt 2 + 2\sqrt 3 } \right)\left( {3 + \sqrt 3 } \right)}}{6}\\ \Leftrightarrow A = - \frac{{6 + 2\sqrt 3 - 3\sqrt 2 - \sqrt 6 + 6\sqrt 3 + 6}}{6}\\ \Leftrightarrow A = - \frac{{12 + 8\sqrt 3 - 3\sqrt 2 - \sqrt 6 }}{6}.\end{array}\)
b)
Đặt \(B = {\sin ^2}{90^o} + {\cos ^2}{120^o} + {\cos ^2}{0^o} - {\tan ^2}60 + {\cot ^2}{135^o}\)
Ta có: \(\left\{ \begin{array}{l}\cos {120^o} = - \cos {60^o}\\\cot {135^o} = - \cot {45^o}\end{array} \right. \Rightarrow \left\{ \begin{array}{l}{\cos ^2}{120^o} = {\cos ^2}{60^o}\\{\cot ^2}{135^o} = {\cot ^2}{45^o}\end{array} \right.\)
\( \Rightarrow B = {\sin ^2}{90^o} + {\cos ^2}{60^o} + {\cos ^2}{0^o} - {\tan ^2}60 + {\cot ^2}{45^o}\)
Sử dụng bảng giá trị lượng giác của một số góc đặc biệt, ta có:
\(\left\{ \begin{array}{l}\cos {0^o} = 1;\;\;\cot {45^o} = 1;\;\;\cos {60^o} = \frac{1}{2}\\\tan {60^o} = \sqrt 3 ;\;\;\sin {90^o} = 1\end{array} \right.\)
\( \Rightarrow B = {1^2} + {\left( {\frac{1}{2}} \right)^2} + {1^2} - {\left( {\sqrt 3 } \right)^2} + {1^2}\)
\( \Leftrightarrow B = 1 + \frac{1}{4} + 1 - 3 + 1 = \frac{1}{4}.\)
c
Đặt \(C = \cos {60^o}.\sin {30^o} + {\cos ^2}{30^o}\)
Sử dụng bảng giá trị lượng giác của một số góc đặc biệt, ta có:
\(\sin {30^o} = \frac{1}{2};\;\;\cos {30^o} = \frac{{\sqrt 3 }}{2};\;\cos {60^o} = \frac{1}{2}\;\)
\( \Rightarrow C = \frac{1}{2}.\frac{1}{2} + {\left( {\;\frac{{\sqrt 3 }}{2}} \right)^2} = \frac{1}{4} + \frac{3}{4} = 1.\)
\(A=tan18^otan288+sin32^osin148^o-sin302^osin122^o\)
\(=tan18^o.tan\left(-72^o\right)+sin32^o.sin32^o+sin58^o.sin58^o\)
\(=-tan18^o.cot18^o+sin^232^o+sin^258^o\)
\(=-1+sin^232^o+cos^232^2=-1+1=0\).
b) \(B=\dfrac{1+sin^4\alpha-cos^4\alpha}{1-sin^6\alpha-cos^6\alpha}\)
\(=\dfrac{1+\left(sin^2\alpha+cos^2\alpha\right)\left(sin^2\alpha-cos^2\alpha\right)}{1-\left(sin^6\alpha+cos^6\alpha\right)}\)
\(=\dfrac{1+sin^2\alpha-cos^2\alpha}{1-\left(sin^2\alpha+cos^2\alpha\right)\left(sin^2\alpha-sin\alpha cos\alpha+cos^2\alpha\right)}\)
\(=\dfrac{sin^2\alpha+1-cos^2\alpha}{1-\left(1-sin\alpha.cos\alpha\right)}\)
\(=\dfrac{sin^2\alpha+sin^2\alpha}{sin\alpha cos\alpha}\)
\(=\dfrac{2sin^2\alpha}{sin\alpha cos\alpha}=\dfrac{2sin\alpha}{cos\alpha}=2tan\alpha\).
a) \(sin20^o+2sin40^o-sin100^o=sin20^o-sin100^o+2sin40^o\)
\(=2cos60^osin\left(-40^o\right)+2sin40^o\)\(=-2cos60^osin40^o+2sin40^o\)
\(=2sin40^o\left(-cos60^o+1\right)=2sin40^o.\left(-\dfrac{1}{2}+1\right)=sin40^o\)(đpcm).
b) \(\dfrac{sin\left(45^o+\alpha\right)-cos\left(45^o+\alpha\right)}{sin\left(45^o+\alpha\right)+cos\left(45^o+\alpha\right)}\)
\(=\dfrac{sin\left(45^o+\alpha\right)-sin\left(45^o-\alpha\right)}{sin\left(45^o+\alpha\right)+sin\left(45^o-\alpha\right)}=\dfrac{2cos45^o.sin\alpha}{2sin45^o.cos\alpha}\)
\(=tan\alpha\) (Đpcm).
\(A = \cos {75^0}\cos {15^0} = \frac{1}{2}\left[ {\cos \left( {{{75}^0} - {{15}^0}} \right) + \cos \left( {{{75}^0} + {{15}^0}} \right)} \right] \\= \frac{1}{2}.\cos {60^0}.\cos {90^0} = 0\)
\(B = \sin \frac{{5\pi }}{{12}}\cos \frac{{7\pi }}{{12}} = \frac{1}{2}\left[ {\sin \left( {\frac{{5\pi }}{{12}} - \frac{{7\pi }}{{12}}} \right) + \sin \left( {\frac{{5\pi }}{{12}} + \frac{{7\pi }}{{12}}} \right)} \right] \\= \frac{1}{2}\sin \left( { - \frac{{2\pi }}{{12}}} \right).\sin \left( {\frac{{12\pi }}{{12}}} \right) = - \frac{1}{2}\sin \frac{\pi }{6}\sin \pi = 0\)
a) \(sin110^ocos130^otan30^ocot320^o\)
Ta có \(sin110^o>0;cos130^o< 0;tan30^o>0;cot320^o< 0\) nên
\(sin110^ocos130^otan30^ocot320^o>0\).
b) \(sin\left(-50^o\right)tan170^ocos\left(-91^o\right)sin530^o\)
\(=-sin50^otan170^o.cos91^osin170^o\)
Do \(sin50^o>0;tan170^o< 0;cos91^o< 0,sin170^o>0\)
nên \(=-sin50^otan170^o.cos91^osin170^o< 0\)
hay \(sin\left(-50^o\right)tan170^ocos\left(-91^o\right)sin530^o< 0\).
a) Ta có :
\(\sin110^0>0;\cos130^0< 0;\tan30^0>0;cot320^0< 0\)
do đó tích của chúng dương.
b) \(\sin\left(-50^0\right)< 0;tan170^0< 0;\cos\left(-91^0\right)< 0;\sin530^0>0\)
do đó tích của chúng âm.
a)\(sin^2\left(180^o-\alpha\right)+tan^2\left(180-\alpha\right).tan^2\left(270^o+\alpha\right)\)\(+sin\left(90^o+\alpha\right)cos\left(\alpha-360^o\right)\)
\(=sin^2\alpha+tan^2\alpha.cot^2\alpha+cos\alpha cos\alpha\)
\(=sin^2\alpha+cos^2\alpha+\left(tan\alpha cot\alpha\right)^2=1+1=2\).
\(\dfrac{cos\left(\alpha-180^o\right)}{sin\left(180^o-\alpha\right)}+\dfrac{tan\left(\alpha-180^o\right)cos\left(180^o+\alpha\right)sin\left(270^o+\alpha\right)}{tan\left(270^o+\alpha\right)}\)
\(=\dfrac{cos\left(180^o-\alpha\right)}{sin\left(180^o-\alpha\right)}+\dfrac{-tan\left(180^o-\alpha\right).cos\alpha.sin\left(90^o+\alpha\right)}{-tan\left(90^o+\alpha\right)}\)
\(=tan\left(180^o-\alpha\right)+\dfrac{tan\alpha.cos\alpha.cos\alpha}{cot\alpha}\)
\(=-tan\alpha+tan^2\alpha cos^2\alpha\)
\(=tan\alpha\left(-1+tan\alpha cos^2\alpha\right)\)
\(=tan\alpha\left(sin\alpha cos\alpha-1\right)\).
\(P=4\left[\left(cos^21^0+cos^289^0\right)+\left(cos^22^0+cos^288^0\right)+...+\left(cos^244^0+cos^246^0\right)+cos^245^0\right]\)
\(=4\left[\left(cos^21^0+sin^21^0\right)+\left(cos^22^0+sin^22^0\right)+...+\left(cos^244^0+sin^244^0\right)+cos^245^0\right]\)
\(=4\left(1+1+...+1+\frac{\sqrt{2}}{2}\right)\)
a, \(cos^2x-cosx=0\)
\(\Leftrightarrow cosx\left(cosx-1\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}cosx=0\\cosx=1\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{\pi}{2}+k\pi\\x=0\end{matrix}\right.\)
b, \(2sin2x+\sqrt{2}sin4x=0\)
\(\Leftrightarrow2sin2x+2\sqrt{2}sin2x.cos2x=0\)
\(\Leftrightarrow sin2x\left(1+\sqrt{2}cos2x\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}sin2x=0\\1+\sqrt{2}cos2x=0\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}2x=k\pi\\cos2x=-\dfrac{\sqrt{2}}{2}\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{k\pi}{2}\\2x=\dfrac{3\pi}{4}+k2\pi\\2x=\dfrac{\pi}{4}+k2\pi\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{k\pi}{2}\\x=\dfrac{3\pi}{8}+k\pi\\x=\dfrac{\pi}{8}+k\pi\end{matrix}\right.\)
a, \(cos^2x-cosx=0\)
\(\Leftrightarrow cosx\left(cosx-1\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}cosx=0\\cosx=1\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{\pi}{2}+k\pi\\x=k2\pi\end{matrix}\right.\) (k ∈ Z)
Vậy...
b, \(2sin2x+\sqrt{2}sin4x=0\)
\(\Leftrightarrow2sin2x+2\sqrt{2}sin2x.cos2x=0\)
\(\Leftrightarrow2sin2x\left(1+\sqrt{2}cos2x\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}sin2x=0\\cos2x=\dfrac{-\sqrt{2}}{2}\end{matrix}\right.\) \(\Leftrightarrow\left[{}\begin{matrix}2x=k\pi\\2x=\pm\dfrac{3\pi}{4}+k2\pi\end{matrix}\right.\) \(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{k\pi}{2}\\x=\pm\dfrac{3\pi}{8}+k\pi\end{matrix}\right.\)
Vậy...
c, \(8cos^2x+2sinx-7=0\)
\(\Leftrightarrow8\left(1-sin^2x\right)+2sinx-7=0\)
\(\Leftrightarrow8sin^2x-2sinx-1=0\)
\(\Leftrightarrow\left[{}\begin{matrix}sinx=\dfrac{1}{2}\\sinx=-\dfrac{1}{4}\end{matrix}\right.\) \(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{\pi}{6}+k2\pi\\x=\dfrac{5\pi}{6}+k2\pi\\x=arcsin\left(-\dfrac{1}{4}\right)+k2\pi\\x=\pi-arcsin\left(-\dfrac{1}{4}\right)+k2\pi\end{matrix}\right.\)
Vậy...
d, \(4cos^4x+cos^2x-3=0\)
\(\Leftrightarrow\left[{}\begin{matrix}cos^2x=\dfrac{3}{4}\\cos^2x=-1\left(loai\right)\end{matrix}\right.\)
\(\Leftrightarrow\dfrac{cos2x+1}{2}=\dfrac{3}{4}\)
\(\Leftrightarrow cos2x=\dfrac{1}{2}\)
\(\Leftrightarrow2x=\pm\dfrac{\pi}{3}+k2\pi\)
\(\Leftrightarrow x=\pm\dfrac{\pi}{6}+k\pi\)
Vậy...
e, \(\sqrt{3}tanx-6cotx+\left(2\sqrt{3}-3\right)=0\) (ĐK: \(x\ne\dfrac{k\pi}{2}\))
\(\Leftrightarrow\sqrt{3}tanx-\dfrac{6}{tanx}+\left(2\sqrt{3}-3\right)=0\)
\(\Leftrightarrow\sqrt{3}tan^2x+\left(2\sqrt{3}-3\right)tanx-6=0\)
\(\Leftrightarrow\left[{}\begin{matrix}tanx=\sqrt{3}\\tanx=-2\end{matrix}\right.\) \(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{\pi}{3}+k\pi\left(tm\right)\\x=arctan\left(-2\right)+k\pi\end{matrix}\right.\)
Vậy...
HS tự làm