tana = 3/4.
=>cota=1/ tana =1:3/4=4/3
sina /cosa =tana
=> sina =tana .cosa =3/4. cosa
lại có sin^2(a)+cos^2(a)=1
<=>9/16cos^2(a)+cos^2=1
<=>25/16cos^2(a)=1
<=>cos^2(a)=16/25
=>[cosa =4/5=>sina =3/5
    [cosa =-4/5=> sina =-2/5

Lung tung hả

\(\sin^2\widehat{A}+\cos^2\widehat{A}=1\Leftrightarrow\cos^2\widehat{A}=1-\left(\dfrac{3}{5}\right)^2=1-\dfrac{9}{25}=\dfrac{16}{25}\\ \Leftrightarrow\cos\widehat{A}=\dfrac{4}{5}\\ \tan\widehat{A}=\dfrac{\sin\widehat{A}}{\cos\widehat{A}}=\dfrac{3}{4}\\ \Rightarrow\cot\widehat{A}=\dfrac{1}{\tan\widehat{A}}=\dfrac{4}{3}\)

Ta có: \(\sin^2A+cos^2A=1\)

=>\(cos^2A=1-\left(\frac35\right)^2=1-\frac{9}{25}=\frac{16}{25}\)

=>\(cosA=\frac45\)

tan A=sin A:cosA

\(=\frac35:\frac45=\frac35\times\frac54=\frac34\)

\(\cot A=1:\frac34=\frac43\)

TA có: \(\sin^2A+cos^2A=1\)

=>\(cos^2A=1-\left(\frac35\right)^2=1-\frac{9}{25}=\frac{16}{25}=\left(\frac45\right)^2\)

=>\(cosA=\frac45\)

tan A=\(\frac{\sin A}{cosA}=\frac35:\frac45=\frac34\)

\(\cot A=\frac{1}{\tan A}=1:\frac34=\frac43\)

Bài 2: 

\(\cos\widehat{A}=\dfrac{3\sqrt{39}}{20}\)

\(\tan\widehat{A}=\dfrac{7}{20}:\dfrac{3\sqrt{39}}{20}=\dfrac{7}{3\sqrt{39}}=\dfrac{7\sqrt{39}}{117}\)

\(\cot\widehat{A}=\dfrac{3\sqrt{39}}{7}\)