# Thread: find the value of sine theta

1. ## find the value of sine theta

2. Originally Posted by aeroflix
Hint: Use an appropriate double angle formula to first get the value of $\sin^2 \theta$.

3. i dont get it double angle.. i know half

4. Originally Posted by aeroflix
i dont get it double angle.. i know half
Go to your textbook or class notes and look up the formulas for $\cos (2 \theta)$.

5. heres my best work... where did i went wrong?

6. Originally Posted by aeroflix
heres my best work... where did i went wrong?
The question asked you to find $\sin \theta$, not solve for $\theta$. You have found $\sin^2 \theta = \frac{4}{13} \Rightarrow \sin \theta = \pm \sqrt{\frac{4}{13}}$. Your job is to decide which of the + or - solutins to keep.

7. but in order to check if u have correct answer, if u try to substitute the theta value to the 1st given equation, it doesnt fit...

is there a problem ?

8. ok i got iot... i was correct.. its the positive side... so should the answer be 0.5546?

9. Originally Posted by aeroflix
but in order to check if u have correct answer, if u try to substitute the theta value to the 1st given equation, it doesnt fit...

is there a problem ?
No, there is no problem. You are obviously making a mistake somewhere. But unless you show what you did and why "it doesnt fit ...." it is impossible to know what mistake yuo have made. My previous reply tells you what to do.