# find the value of sine theta

• September 3rd 2010, 05:47 AM
aeroflix
find the value of sine theta
• September 3rd 2010, 05:49 AM
mr fantastic
Quote:

Originally Posted by aeroflix

Hint: Use an appropriate double angle formula to first get the value of $\sin^2 \theta$.
• September 3rd 2010, 05:55 AM
aeroflix
i dont get it double angle.. i know half
• September 3rd 2010, 05:58 AM
mr fantastic
Quote:

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 $\cos (2 \theta)$.
• September 3rd 2010, 06:08 AM
aeroflix
heres my best work... where did i went wrong?Attachment 18816
• September 3rd 2010, 06:15 AM
mr fantastic
Quote:

Originally Posted by aeroflix
heres my best work... where did i went wrong?Attachment 18816

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.
• September 3rd 2010, 06:17 AM
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 ?
• September 3rd 2010, 06:19 AM
aeroflix
ok i got iot... i was correct.. its the positive side... so should the answer be 0.5546?
• September 3rd 2010, 06:23 AM
mr fantastic
Quote:

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.