# Thread: [Trig] Double-Angle and Half-Angle

1. ## [Trig] Double-Angle and Half-Angle

I'm having a lot of problems with questions that involve double angles and half angles... Can someone help me understand how to solve two types of problems?

The first one is: ~Use the following information to find sin 2A, cos 2A, and tan 2A. cos A = 5/13, in quadrant 4.~

The second one is: ~Use the following information to find cos A/2, sin A/2, and tan A/2. There is a right triangle and the leg opposite angle A is 8 and the adjacent leg to angle A is 15.~

There are a lot of questions like these two in my homework. Can someone please tell me how to do them? I have all the formulas in my text book, but I still don't understand how to do it.

2. Originally Posted by Luizzmp
Use the following information to find sin 2A, cos 2A, and tan 2A. cos A = 5/13, in quadrant 4.
Apply double-angle formulas.

Originally Posted by Luizzmp
Use the following information to find cos A/2, sin A/2, and tan A/2. There is a right triangle and the leg opposite angle A is 8 and the adjacent leg to angle A is 15.
Use the Pythagorean Theorem to find the length of the remaining side. Find the sine, cosine, and tangent of the angle. Then apply half-angle formulas.

3. Originally Posted by stapel
Apply double-angle formulas.

Use the Pythagorean Theorem to find the length of the remaining side. Find the sine, cosine, and tangent of the angle. Then apply half-angle formulas.

Can you show me where to apply the formulas though? At least for the sin in both problems and I'll figure out how to do it for cos and tan

Yeah. Thats what I did first for the second problem. The Hypotenuse of the triangle is 17, so sin A= 8/17, cos A= 15/17, and tan A= 8/15... but then again, I don't know how to apply the formulas there xD

4. Originally Posted by Luizzmp
I'm having a lot of problems with questions that involve double angles and half angles... Can someone help me understand how to solve two types of problems?

The first one is: ~Use the following information to find sin 2A, cos 2A, and tan 2A. cos A = 5/13, in quadrant 4.~

If cos(A) = 5/13 → sin(A) = -12/13 → sin(2A) = 2 * sin(A) * cos(A) = ???

The second one is: ~Use the following information to find cos A/2, sin A/2, and tan A/2. There is a right triangle and the leg opposite angle A is 8 and the adjacent leg to angle A is 15.~

There are a lot of questions like these two in my homework. Can someone please tell me how to do them? I have all the formulas in my text book, but I still don't understand how to do it.
.

5. Originally Posted by Subhotosh Khan
If cos(A) = 5/13 → sin(A) = -12/13 → sin(2A) = 2 * sin(A) * cos(A) = ???.

ohhh ._. I think I get it now. Thank you Subhotosh I'll try to do the other ones with double angles now...

Is there any chance you could show me how to do one with half angles too?

6. Originally Posted by Luizzmp
ohhh ._. I think I get it now. Thank you Subhotosh I'll try to do the other ones with double angles now...

Is there any chance you could show me how to do one with half angles too?
Do you know the half-angle formulae? What are those?

7. Originally Posted by Subhotosh Khan
Do you know the half-angle formulae? What are those?
I don't remember xD but I am finished with that homework 8) I figured out how to do it with a pm someone from the forum sent me with an example ;3