Multiples And Sub-Multiples
We know,
\(
\sin 3A = \sin (2A + A) = \sin 2A.\,\cos A + \cos 2A\,\,.\sin A
\)
= 2 sin A cosA cosA + (1- 2sin2A) . sinA
= 2sin (1-sin2A)+ (1-2sin2A) . sinA
sin3A = 3sinA -4sin3A ...... (v)
We know,
cos3A = cos(2A+A) = cos2A . cosA - sin2A. sinA
= (2cos2A - 1) cosA - 2sinA . cosA . sinA
= 2cos3A - cosA - 2(1-cos2A) . cosA
= 2 cos3A - cosA - 2cosA + 2cos3A
cos3A = 4cos3A - 3cosA .........(vi)
We know
tan 3A = tan(2A + A)
\(
= \frac{{\tan 2A + \tan A}}
{{1 - \tan 2A.\,\tan A}}
\)
\(
= \frac{{\frac{{2\tan A}}
{{1 - \tan ^2 A}} + \tan A}}
{{1 - \left( {\frac{{2\tan A}}
{{1 - \tan ^2 A}}} \right)\tan A}}
\)
\(
= \frac{{\frac{{2\tan A + \tan (1 - \tan ^2 A)}}
{{1 - \tan ^2 A}}}}
{{\frac{{(1 - \tan ^2 A) - 2\tan ^2 A}}
{{1 - \tan ^2 A}}}}
\)
\(
\tan 3A = \frac{{3\tan A - \tan ^3 A}}
{{1 - 3\tan ^2 A}}\,\,\,\,\,\,\, \to \,\,(vii)
\)
We know
cot3A = cot(2A+A) =\(
\frac{{\cot 2A.\cot A - 1}}
{{\cot 2A + \cot A}}
\)
=\(
\frac{{\left( {\frac{{\cot ^2 A - 1}}
{{2\cot A}}} \right).\cot A - 1}}
{{\left( {\frac{{\cot ^2 A - 1}}
{{2\cot A}}} \right) + \cot A}}
\)
=\(
\frac{{\cot ^3 A - 3\cot A}}
{{3\cot ^2 A - 1}}
\)
\(
\cot 3A = \frac{{3\cot A - \cot ^3 A}}
{{1 - 3\cot ^2 A}}\,\,\,\,\,\,\,\,\,\, \to \,\,(viii)
\)