Table of Contents
Trigonometric ratios of multiple of Angle-2A
If A an angle, then its integral multiples 2A, 3A, 4A….. are called multiple angles of A and the multiple angles of A
Trigonometric ratios of Multiple angles of A Formulae
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Sin 2A = 2Sin A cos A
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cos 2A = $cos^2$ A - $sin^2$ A = 2 $cos^2$ A-1 = 1- $sin^2$ A
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tan 2A = $\frac{2tan\ A}{1-tan^2\ A}$
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cot 2A = $\frac{cot^2\ A-1}{2cot\ A}$
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Sin 2A = $\frac{2tan\ A}{1+tan^2\ A}$
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cos 2A = $\frac{1-tan^2\ A}{1+tan^2\ A}$
1) Sin 2A = 2Sin A cos A
Proof:
We have a compound angle formula:
Sin(A+B)= sin A cos B + cos A sin B
If A=B
Sin 2A = sin A cos A + cos A sin A
Sin 2A = 2sin A cos A
2) cos 2A = $cos^2$ A - $sin^2$ A = 2$cos^2$ A-1 = 1- $sin^2$ A
Proof:
i) We have a compound angle formula:
Cos(A+B)= Cos A cos B- Sin A Sin B
If A= B
Cos 2A = $cos^2$ A - $sin^2$ A
ii) Using the trigonometric identity $cos^2$ A + $sin^2$ A =1 in Cos 2A = $cos^2$ A - $sin^2$ A
Cos 2A = (1-$sin^2$ A)-$sin^2$ A
Cos 2A = 1-2$sin^2$ A
iii) Similarly Using the trigonometric identity $cos^2$ A + $sin^2$ A =1 in Cos 2A = $cos^2$ A - $sin^2$ A
Cos 2A = $cos^2$ A - $sin^2$ A
Cos 2A = $cos^2$ A - (1-$cos^2$ A)
Cos 2A = 2 $cos^2$ A -1
Therefore cos 2A = $cos^2$ A - $sin^2$ A = 2$cos^2$ A-1 = 1- $sin^2$ A
3) tan 2A = $\frac{2tan\ A}{1-tan^2\ A}$
Proof:
tan 2A = $\frac{sin\ 2A}{cos\ 2A}$
Using the multiple angle formulae Sin 2A=2Sin A cos A and cos 2A = $cos^2$ A - $sin^2$ A
tan 2A = $\frac{2Sin\ A cos\ A}{cos^2\ A - sin^2\ A }$
Divide $cos^2$ A on both numerator and denominator.
tan 2A = $\frac{\frac{2Sin\ A cos\ A}{cos^2\ A}}{\frac{cos^2\ A - sin^2\ A}{cos^2\ A }}$
tan 2A = $\frac{2tan\ A}{1-tan^2\ A}$
4) cot 2A = $\frac{cot^2\ A-1}{2cot\ A}$
Proof:
Cot 2A = $\frac{1}{tan\ 2A}$
Using tan 2A = $\frac{2tan\ A}{1-tan^2\ A}$ formula
Cot 2A = $\frac{1-tan^2\ A}{2tan\ A}$
= $\frac{1-\frac{1}{cot^2\ A}}{2\frac{1}{cot\ A}}$
cot 2A = $\frac{cot^2\ A-1}{2cot\ A}$
5) Sin 2A = $\frac{2tan\ A}{1+tan^2\ A}$
Proof:*
We have a trigonometric multiple angle formula:
Sin 2A=2Sin A cos A
Use trigonometric identity $cos^2$ A + $sin^2$ A =1
Sin 2A = $\frac{2Sin\ A cos\ A}{cos^2\ A + sin^2\ A}$
Divide with $cos^2$ A on numerator and denominator
Sin 2A = $\frac{\frac{2Sin\ A cos\ A}{cos^2\ A}}{\frac{cos^2\ A + sin^2\ A}{cos^2\ A} }$
Sin 2A = $\frac{2tan\ A}{1+tan^2\ A}$
6) cos 2A = $\frac{1-tan^2\ A}{1+tan^2\ A}$
Proof:
We have a trigonometric multiple angle formula:
cos 2A = $cos^2$ A - $sin^2$ A
Use trigonometric identity $cos^2$ A + $sin^2$ A =1
Cos 2A = $\frac{cos^2\ A-sin^2\ A}{cos^2\ A+sin^2\ A}$
Divide with $cos^2$ A on numerator and denominator
Cos 2A = $\frac{\frac{cos^2\ A-sin^2\ A}{cos^2\ A}}{\frac{cos^2\ A+sin^2\ A}{cos^\ A}}$
Cos 2A = $\frac{1-tan^2\ A}{1+tan^2\ A}$
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