We will learn about the trigonometric ratios of angle \(\frac{A}{2}\) in terms of angle A.
How to express sin A, cos A and tan A in terms of \(\frac{A}{2}\)?
(i) For all values of the angle A we know that, sin 2A = 2 sin A cos A
Now replacing A by \(\frac{A}{2}\) in the above relation then we obtain the relation as,
sin A = 2 sin \(\frac{A}{2}\) cos \(\frac{A}{2}\)
(ii) For all values of the angle A we know that, cos 2A = cos\(^{2}\) A – sin\(^{2}\) A
Now replacing A by \(\frac{A}{2}\) in the above relation then we obtain the relation as,
cos A = cos\(^{2}\) \(\frac{A}{2}\) – sin\(^{2}\) \(\frac{A}{2}\)
(iii) For all values of the angle A we know that, cos 2A = 2 cos\(^{2}\) A  1 or 1 + cos 2A = 2 cos\(^{2}\) A
Now replacing A by \(\frac{A}{2}\) in the above relation then we obtain the relation as,
cos A = 2 cos\(^{2}\) \(\frac{A}{2}\)  1 or 1 + cos A = 2 cos\(^{2}\) \(\frac{A}{2}\)
(iv) For all values of the angle A we know that, cos 2A = 1  2 sin\(^{2}\) A or 1  cos 2A = 2 sin\(^{2}\) A
Now replacing A by \(\frac{A}{2}\) in the above relation then we obtain the relation as,
cos A = 1  2 sin\(^{2}\) \(\frac{A}{2}\) or 1  cos A = 2 sin\(^{2}\) \(\frac{A}{2}\)
(v) For all values of the angle A we know that, tan 2A = 2 tan A/1 – tan^2 A
Now replacing A by A/2 in the above relation then we obtain the relation as,
tan A = \(\frac{2 tan
\frac{A}{2}}{1  tan^{2} \frac{A}{2}}\)
(vi) For all values of the angle A we know that, sin 2A = 2 tan A/1 + tan^2 A
Now replacing A by A/2 in the above relation then we obtain the relation as,
sin A = \(\frac{2 tan \frac{A}{2}}{1 + tan^{2} \frac{A}{2}}\)
(vii) For all values of the angle A we know that, cos 2A = 1  tan^2 A /1 + tan^2 A
Now replacing A by A/2 in the above relation then we obtain the relation as,
cos A = \(\frac{1  tan^{2} \frac{A}{2}}{1 + tan^{2} \frac{A}{2}}\)
Note: Formulas of trigonometric ratios of angle A in terms of angle \(\frac{A}{2}\) is also known as submultiple angle.
`11 and 12 Grade Math
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