How to Solve Trigonometric Functions When \( \cos(t) = -\frac{9}{10} \)?

In summary, the values of cos(2t), sin(2t), cos($\frac{t}{2}$), and sin($\frac{t}{2}$) when cos(t)=$\frac{-9}{10}$ and $\pi$ <t<$\frac{3\pi}{2}$ are:cos(2t)=$\frac{31}{50}$sin(2t)=$\frac{9\sqrt{19}}{50}$cos($\frac{t}{2}$)=$\frac{-\sqrt{5}}{10}$sin($\frac{t}{2}$)=$\frac{\sqrt{95
  • #1
lastochka
29
0
Hello,

I am trying to solve this. This material is not covered in my class, but I still want to know how to do it.
If cos(t)=$\frac{-9}{10}$ where $\pi$ <t<$\frac{3\pi}{2}$ find the values of

cos(2t)=
sin(2t)=
cos($\frac{t}{2}$)=
sin($\frac{t}{2}$)=

Give exact answers, do not use decimal numbers. The answer should be a fraction or an arithmetic expression.

I have no idea how to even start with it. I will appreciate your help!
 
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  • #2
lastochka said:
Hello,

I am trying to solve this. This material is not covered in my class, but I still want to know how to do it.
If cos(t)=$\frac{-9}{10}$ where $\pi$ <t<$\frac{3\pi}{2}$ find the values of

cos(2t)=
sin(2t)=
cos($\frac{t}{2}$)=
sin($\frac{t}{2}$)=

Give exact answers, do not use decimal numbers. The answer should be a fraction or an arithmetic expression.

I have no idea how to even start with it. I will appreciate your help!

You are expected to use the following identities:

$\displaystyle \begin{align*} \sin^2{(x)} + \cos^2{(x)} &\equiv 1 \\ \\ \cos{(2x)} &\equiv \cos^2{(x)} - \sin^2{(x)} \\ &\equiv 2\cos^2{(x)} - 1 \\ &\equiv 1 - 2\sin^2{(x)} \\ \\ \sin{(2x)} &\equiv 2\sin{(x)}\cos{(x)} \end{align*}$

See what you can do...
 
  • #3
Prove It said:
You are expected to use the following identities:

$\displaystyle \begin{align*} \sin^2{(x)} + \cos^2{(x)} &\equiv 1 \\ \\ \cos{(2x)} &\equiv \cos^2{(x)} - \sin^2{(x)} \\ &\equiv 2\cos^2{(x)} - 1 \\ &\equiv 1 - 2\sin^2{(x)} \\ \\ \sin{(2x)} &\equiv 2\sin{(x)}\cos{(x)} \end{align*}$

See what you can do...

Well, I know these identities, but as I said I am not sure how to use them in this question. Can you elaborate? Thanks
 
  • #4
I finally did this problem and the answers are correct. Posting just in case anyone else is interested


cos(2t)=$\frac{31}{50}$

sin(2t)=$\frac{9\sqrt{19}}{50}$

cos(t2)=$\frac{-\sqrt{5}}{10}$

sin(t2)=$\frac{\sqrt{95}}{10}$
 
  • #5
This is how I would work the problem...we know $t$ is in quadrant III, and so sine and cosine of $t$ are both negative.

\(\displaystyle \sin(t)=-\sqrt{1-\left(-\frac{9}{10}\right)^2}=-\sqrt{\frac{19}{100}}=-\frac{\sqrt{19}}{10}\)

Now, we find:

\(\displaystyle \cos(2t)=\frac{81}{100}-\frac{19}{100}=\frac{62}{100}=\frac{31}{50}\)

\(\displaystyle \sin(2t)=2\left(-\frac{\sqrt{19}}{10}\right)\left(-\frac{9}{10}\right)=\frac{9\sqrt{19}}{50}\)

\(\displaystyle \cos\left(\frac{t}{2}\right)=-\sqrt{\frac{1-\frac{9}{10}}{2}}=-\frac{1}{2\sqrt{5}}\)

\(\displaystyle \sin\left(\frac{t}{2}\right)=\sqrt{\frac{1+\frac{9}{10}}{2}}=\frac{\sqrt{19}}{2\sqrt{5}}\)
 

FAQ: How to Solve Trigonometric Functions When \( \cos(t) = -\frac{9}{10} \)?

1. What are the basic trigonometric functions and how are they related?

The basic trigonometric functions are sine, cosine, and tangent. They are related through the unit circle, where the sine is the y-coordinate, cosine is the x-coordinate, and tangent is the ratio of the two sides of a right triangle formed by the angle.

2. How do you solve a trigonometric function problem?

To solve a trigonometric function problem, you need to use the given information to find the missing side or angle of a triangle. This can be done using the trigonometric ratios and inverse trigonometric functions.

3. What is the unit circle and why is it important in trigonometry?

The unit circle is a circle with a radius of 1, centered at the origin of a coordinate plane. It is important in trigonometry because it allows us to easily visualize and understand the relationships between trigonometric functions and angles.

4. How do you use trigonometric functions to find the area of a triangle?

To find the area of a triangle using trigonometric functions, you can use the formula A = 1/2 * b * h, where b is the base of the triangle and h is the height. You can find these values by using trigonometric ratios and the given angles or sides of the triangle.

5. What are some real-life applications of trigonometric functions?

Trigonometric functions have many real-life applications, such as in navigation, engineering, and astronomy. They can be used to calculate distances and angles, determine heights and distances of objects, and predict the movement of celestial bodies.

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