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December 21, 2014

December 21, 2014

Posted by **Anonymous** on Wednesday, November 21, 2007 at 6:29pm.

3sinx = 2cos^{2}x

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I don't know how to solve this equation...this is what I have, but I don't know if I'm on the right track or not

3sinx = 1 - 2sin^{2}x

3sinx - 1 + 2sin^{2}x = 0

2sin^{2}x + 3sinx - 1 = 0

sinx(3 + 2sinx) = 1

...and if I am, what do I do next now?

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The answers are:

30 and 150 degrees

- Math - Solving Trig Equations -
**Michael**, Wednesday, November 21, 2007 at 6:35pm3sinx = 2cos^2(x)

I believe you've treated this as cos(2x) and expanded it as a double-angle identity. However, you can't do this because the cosine function is being squared. The 2 is not being multiplied by the x.

I'm working on it...

- Math - Solving Trig Equations -
**Michael**, Wednesday, November 21, 2007 at 6:43pmStart by recalling the most important identity. My math teacher calls this "the #1 Identity."

sin^2(x) + cos^2(x) = 1

We want to simplify our trig equation by writing everything in terms of sine. Let's solve the #1 Identity for cos^2(x) because we have that in our trig equation.

cos^2(x) = 1 - sin^2(x)

Plug that into the trig equation, and see what you can get from there. Let me know if you get stuck along the way.

- Math - Solving Trig Equations -
- Math - Solving Trig Equations -
**Anonymous**, Wednesday, November 21, 2007 at 6:49pmwow I forgot about that identity, thank you!

But, I'm still stuck...

I get

cos^2x + sin^2x = 1

I can't use any compound identities for this...I don't know how to isolate for either cos or sin...

- Math - Solving Trig Equations -
**Michael**, Wednesday, November 21, 2007 at 6:57pmThe only identity you need it the #1 Identity.

Plugging in 1 - sin^2(x) for cos^2(x), you should get...

3sinx = 2(1 - sin^2(x))

Use the distributive property, collect everything on the left, and then factor.

- Math - Solving Trig Equations -
**Michael**, Wednesday, November 21, 2007 at 6:57pmThe only identity you need is the #1 Identity.

Plugging in 1 - sin^2(x) for cos^2(x), you should get...

3sinx = 2(1 - sin^2(x))

Use the distributive property, collect everything on the left, and then factor.

- Math - Solving Trig Equations -
- Math - Solving Trig Equations -
**Anonymous**, Wednesday, November 21, 2007 at 7:09pmthanks, I solved it :)

- Math - Solving Trig Equations -
**Michael**, Wednesday, November 21, 2007 at 7:38pmGlad to be of service. :)

- Math - Solving Trig Equations -

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