Take the 2-minute tour ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

Please help me to solve this problem on how to proof if it is a double angle

$$\begin{align*} \cos 2 A &= 1-2\sin^2A\\ \sin 2 A &= 2\sin A\cos A\\ \tan 2A &=2\sin A\cos A\\ \end{align*}$$

share|improve this question
    
do you know the formula of $\sin(A+B), \cos (A+B)$ ? –  Santosh Linkha Jan 10 '13 at 23:03
    
Do you have access to the angle sum identities? If so, start with an angle sum of an angle with itself. (e.g., $\cos(\alpha + \alpha)$. –  Todd Wilcox Jan 10 '13 at 23:04
    
@user335160: Welcome to MSE! Is this homework? If so, it should be tagged as such. What have you tried? Where are you confused? These sorts of additions to questions help to MSE community to better help you with questions. Regards! –  Amzoti Jan 10 '13 at 23:05
    
sorry I don't know because I am new to this. please help me to solve this problem with solution. thank you very much –  user335160 Jan 10 '13 at 23:06
    
It would help us help you if you answer the first two questions. –  amWhy Jan 10 '13 at 23:07

1 Answer 1

Exists two basic identities for sum of angles and they are $$\sin(A+B)=\sin A\cos B+\sin B\cos A$$ $$\cos(A+B)=\cos A\cos B-\sin A\sin B$$ if in these you put $A=B$ you get $$\sin(2A)=2\sin A\cos A$$ $$\cos(2A)=\cos^2 A-\sin^2 A$$ then you need to manipulate following identities $$\tan A=\frac{\sin A}{\cos A}$$ $${\sin^2 A}+{\cos^2 A}=1$$

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.