\(a,(\frac{2a-1}{2a+1}-\frac{2a-3}{2a-1}):\frac{2a-1}{2a+1}\)\(\)
K
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AH
Akai Haruma
Giáo viên
30 tháng 4 2019
Lời giải:
a)
\(\frac{\sin ^2a+2\cos ^2a-1}{\cot ^2a}=\frac{(\sin ^2a+\cos ^2a)+\cos ^2a-1}{\cot ^2a}=\frac{1+\cos ^2a-1}{\cot ^2a}=\frac{\cos ^2a}{\cot ^2a}=\frac{\cos ^2a}{(\frac{\cos a}{\sin a})^2}=\sin ^2a\)
b)
\(\frac{1-\sin ^2a\cos ^2a}{\cos ^2a}-\cos ^2a=\frac{1}{\cos ^2a}-\sin ^2a-\cos ^2a\)
\(=\frac{\sin ^2a+\cos ^2a}{\cos ^2a}-(\sin ^2a+\cos ^2a)=\tan ^2a+1-1=\tan ^2a\)
c)
\(\frac{\sin ^2a-\tan ^2a}{\cos ^2a-\cot ^2a}=\frac{\sin ^2a-\frac{\sin ^2a}{\cos ^2a}}{\cos ^2a-\frac{\cos ^2a}{\sin ^2a}}=\frac{\sin ^4a(\cos ^2a-1)}{\cos ^4a(\sin ^2a-1)}\)
\(=\frac{\sin ^4a(-\sin ^2a)}{\cos ^4a(-\cos ^2a)}=\frac{\sin ^6a}{\cos ^6a}=\tan ^6a\)
NV
Nguyễn Việt Lâm
Giáo viên
28 tháng 11 2019
\(\frac{cosa}{1+sina}+\frac{sina}{cosa}=\frac{cos^2a+sina\left(1+sina\right)}{cosa\left(1+sina\right)}=\frac{1+sina}{cosa\left(1+sina\right)}=\frac{1}{cosa}\)
\(\frac{sin^2a+cos^2a+2sina.cosa}{\left(sina-cosa\right)\left(sina+cosa\right)}=\frac{\left(sina+cosa\right)^2}{\left(sina-cosa\right)\left(sina+cosa\right)}=\frac{sina+cosa}{sina-cosa}=\frac{\frac{sina}{cosa}+1}{\frac{sina}{cosa}-1}=\frac{tana+1}{tana-1}\)
\(\left(sin^2a\right)^3+\left(cos^2a\right)^3=\left(sin^2a+cos^2a\right)^3-3sin^2a.cos^2a\left(sin^2a+cos^2a\right)\)
\(=1-3sin^2a.cos^2a\)
\(sin^2a-tan^2a=tan^4a\left(\frac{sin^2a}{tan^4a}-\frac{1}{tan^2a}\right)=tan^4a\left(sin^2a.\frac{cos^2a}{sin^2a}-\frac{1}{tan^2a}\right)\)
\(=tan^4a\left(cos^2a-cot^2a\right)\) bạn ghi sai đề câu này
\(\frac{tan^3a}{sin^2a}-\frac{1}{sina.cosa}+\frac{cot^3a}{cos^2a}=tan^3a\left(1+cot^2a\right)-\frac{1}{sina.cosa}+cot^3a\left(1+tan^2a\right)\)
\(=tan^3a+tana-\frac{1}{sina.cosa}+cot^3a+cota\)
\(=tan^3a+cot^3a+\frac{sina}{cosa}+\frac{cosa}{sina}-\frac{1}{sina.cosa}\)
\(=tan^3a+cot^3a+\frac{sin^2a+cos^2a-1}{sina.cosa}=tan^3a+cot^3a\)
HQ
Hà Quang Minh
Giáo viên
21 tháng 9 2023
a) Vì \(\frac{\pi }{2} < a < \pi \) nên \(\cos a < 0\)
Ta có: \({\sin ^2}a + {\cos ^2}a = 1\)
\(\Leftrightarrow \frac{1}{9} + {\cos ^2}a = 1\)
\(\Leftrightarrow {\cos ^2}a = 1 - \frac{1}{9}= \frac{8}{9}\)
\(\Leftrightarrow \cos a =\pm\sqrt { \frac{8}{9}} = \pm \frac{{2\sqrt 2 }}{3}\)
Vì \(\cos a < 0\) nên \(cos a =-\frac{{2\sqrt 2 }}{3}\)
Suy ra \(\tan a = \frac{{\sin a}}{{\cos a}} = \frac{{\frac{1}{3}}}{{ - \frac{{2\sqrt 2 }}{3}}} = - \frac{{\sqrt 2 }}{4}\)
Ta có: \(\sin 2a = 2\sin a\cos a = 2.\frac{1}{3}.\left( { - \frac{{2\sqrt 2 }}{3}} \right) = - \frac{{4\sqrt 2 }}{9}\)
\(\cos 2a = 1 - 2{\sin ^2}a = 1 - \frac{2}{9} = \frac{7}{9}\)
\(\tan 2a = \frac{{2\tan a}}{{1 - {{\tan }^2}a}} = \frac{{2.\left( { - \frac{{\sqrt 2 }}{4}} \right)}}{{1 - {{\left( { - \frac{{\sqrt 2 }}{4}} \right)}^2}}} = - \frac{{4\sqrt 2 }}{7}\)
b) Vì \(\frac{\pi }{2} < a < \frac{{3\pi }}{4}\) nên \(\sin a > 0,\cos a < 0\)
\({\left( {\sin a + \cos a} \right)^2} = {\sin ^2}a + {\cos ^2}a + 2\sin a\cos a = 1 + 2\sin a\cos a = \frac{1}{4}\)
Suy ra \(\sin 2a = 2\sin a\cos a = \frac{1}{4} - 1 = - \frac{3}{4}\)
Ta có: \({\sin ^2}a + {\cos ^2}a = 1\;\)
\( \Leftrightarrow \left( {\frac{1}{2} - {\cos }a} \right)^2 + {\cos ^2}a - 1 = 0\)
\( \Leftrightarrow \frac{1}{4} - \cos a + {\cos ^2}a + {\cos ^2}a - 1 = 0\)
\( \Leftrightarrow 2{\cos ^2}a - \cos a - \frac{3}{4} = 0\)
\( \Rightarrow \cos a = \frac{{1 - \sqrt 7 }}{4}\) (Vì \(\cos a < 0)\)
\(\cos 2a = 2{\cos ^2}a - 1 = 2.{\left( {\frac{{1 - \sqrt 7 }}{4}} \right)^2} - 1 = - \frac{{\sqrt 7 }}{4}\)
\(\tan 2a = \frac{{\sin 2a}}{{\cos 2a}} = \frac{{ - \frac{3}{4}}}{{ - \frac{{\sqrt 7 }}{4}}} = \frac{{3\sqrt 7 }}{7}\)
MN
0
11 tháng 10 2020
Trước hết xoá \(\frac{2x}{a^2-a+1}\)ở 2 vế. Nếu \(\frac{a}{a+1}>0\left(a< -1;a>0\right)\)thì \(x< \frac{a}{4}\). Nếu \(\frac{a}{a+1}< 0\left(-1< a< 0\right)\)thì \(x>\frac{a}{4}\)
KN
12 tháng 10 2020
\(ĐKXĐ:a\ne-1\)
\(\frac{2x}{a^2-a+1}-\frac{1}{2a+2}< \frac{4x-1}{2a^2-2a+2}+\frac{a-2ax}{1+a^3}\Leftrightarrow\frac{2x}{a^2-a+1}-\frac{1}{2a+2}< \frac{2x}{a^2-a+1}-\frac{1}{2a^2-2a+2}+\frac{a}{1+a^3}-\frac{2ax}{1+a^3}\)\(\Leftrightarrow\frac{1}{2a+2}-\frac{1}{2a^2-2a+2}+\frac{a}{1+a^3}>\frac{2ax}{1+a^3}\Leftrightarrow\frac{a^2-a+1-a-1+2a}{2\left(a^3+1\right)}>\frac{2ax}{1+a^3}\Leftrightarrow\frac{a^2}{2\left(1+a^3\right)}>\frac{4ax}{2\left(1+a^3\right)}\)\(\Leftrightarrow\frac{4ax}{a+1}< \frac{a^2}{a+1}\)
* Nếu \(\frac{a}{a+1}>0\)(tức là a < -1 hoặc a > 0) thì \(x< \frac{a}{4}\)
* Nếu \(\frac{a}{a+1}< 0\)(tức là -1 < a < 0) thì \(x>\frac{a}{4}\)
ĐKXĐ: \(a\ne\pm\frac{1}{2}\)
\(\left(\frac{2a-1}{2a+1}-\frac{2a-3}{2a-1}\right):\frac{2a-1}{2a+1}\)
\(=\left(\frac{\left(2a-1\right)^2}{\left(2a+1\right)\left(2a-1\right)}-\frac{\left(2a-3\right)\left(2a+1\right)}{\left(2a-1\right)\left(2a+1\right)}\right).\frac{2a+1}{2a-1}\)
\(=\left(\frac{4a^2-4a+1}{\left(2a+1\right)\left(2a-1\right)}-\frac{4a^2-4a+3}{\left(2a+1\right)\left(2a-1\right)}\right).\frac{2a+1}{2a-1}\)
\(=\frac{-2}{\left(2a+1\right)\left(2a-1\right)}.\frac{2a+1}{2a-1}=\frac{-2}{\left(2a-1\right)^2}\)