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October 20, 2014

October 20, 2014

Posted by **Priya** on Thursday, May 1, 2008 at 4:46pm.

Yes, thank you again Reiny!

My next question is to find sin theta and cos theta.

cos 2theta = 1/3 and pi less than or equal to 2theta less than or equal to 2pi

I hope Im not overburdening you with question:/

- trig -
**Reiny**, Thursday, May 1, 2008 at 5:04pm<<I hope Im not overburdening you with question:/>>

lol, I love trig and do these kind of questions for recreation.

for this one you will need to know

1. sin2A = 2sinAcosA, and

2. cos2A = cos^2 A - sin^2 A

or 2cos^2 A -1

or 1 - 2sin^2 A

I will do the first, you do the second

cos2A = 1-2sin^2A

1/3 = 1 - 2sin^2 A

2 sin^2 A = 1 - 1/3

sin^2 A = 1/3

sinA = ±1/√3 (I used A instead of theta)

Can we use both ± ?

2theta is in either I or IV

then theta is in either I or II, but the sine in both of these is positive, so

sinA = +1/√3 only.

you try the second one.

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