# Thread: Easy Trigonometry (5 questions)

1. ## Easy Trigonometry (5 questions)

Solve each equation on the interval [0,2pi]

1) sin^2x-2cosx-2=0

2) 4cos^x=5-4sinx

3) sin2x=cosx

4) cos2x=cosx

5) cos2x+5cosx+3=0

2. ## Re: Easy Trigonometry (5 questions)

Originally Posted by LillyJackson06
Solve each equation on the interval [0,2pi]

1) sin^2x-2cosx-2=0

2) 4cos^x=5-4sinx

3) sin2x=cosx

4) cos2x=cosx

5) cos2x+5cosx+3=0
Please show what work you have been able to do. We are not a homework service.

-Dan

3. ## Re: Easy Trigonometry (5 questions)

I do not know how to start these equations, and what to substitute in, which is the first step, which i need help on.

4. ## Re: Easy Trigonometry (5 questions)

Originally Posted by LillyJackson06
Solve each equation on the interval [0,2pi]
1) sin^2x-2cosx-2=0
2) 4cos^x=5-4sinx
3) sin2x=cosx
4) cos2x=cosx
5) cos2x+5cosx+3=0

Write #1 as $\displaystyle 1-\cos^2(x)-2\cos(x)-2=\cos^2(x)+2\cos(x)+1=0$.
Then let $\displaystyle u=\cos(x)$ solve $\displaystyle u^2+2u+1=0$.

You have left something out of #2.

#3 $\displaystyle \sin(2x)=2\sin(x)\cos(x)$

#5 $\displaystyle \cos(2x)=2\cos^2(x)-1$

5. ## Re: Easy Trigonometry (5 questions)

I now understand #1.

Sorry, #2 should be 4cos^2x=5-4sinx.

I also now get #3 and #5.

Thanks so much.

6. ## Re: Easy Trigonometry (5 questions)

Originally Posted by LillyJackson06
I now understand #1.

Sorry, #2 should be 4cos^2x=5-4sinx.

I also now get #3 and #5.

#2 $\displaystyle 4(1-\sin^2(x))=5-4\sin(x)$