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January 28, 2015

January 28, 2015

Posted by **Anonymous** on Sunday, January 3, 2010 at 12:31am.

2sin^2(5x)+sin(5x)-1=0

- Trigonometry -
**Reiny**, Sunday, January 3, 2010 at 9:21amFor the first, take out a common factor of sin(2x) to get

sin(2x)[cot^2(3x) - 1] = 0

now you have a difference of squares for

sin(2x)(cot(3x - 1)(cot(3x) + 1) = 0

Can you take it from there?

I will do the second one:

Your equation is a quadratic which factors.

(2sin(5x) - 1)(sin(5x) + 1) = 0

sin(5x) = 1/2 or sin(5x) = -1

5x = 30º or 150º or 5x = 270º

x = 6º , 30º , 54º

I don't know what your domain was, but the period of sin(5x) is 360/5º or 72º

so by adding or subtracting multiples of 72 to each of my answers will yield as many answers as you want.

general solutions in radians :

pi/30 + k(2pi/5)

pi/6 + k(2pi/5)

3pi/10 + k(2pi/5) , where k is an integer.

- Trigonometry -
**Anonymous**, Sunday, January 3, 2010 at 1:35pmThank you! Would you mind explaining to me the k(2pi/5) thing? How do you know when to use that and how do you know when to use just k(pi/5) without the 2? Is the period always going to be the number in front of x (like in this case it was 5 because of 5x)?

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