# trig equations

• Dec 5th 2012, 08:08 AM
enea54
trig equations
hi guys , i am a high school student , and i have a trigonometry test tomorrow . i would appreciate every help from you.
for now i can't solve these trigonometry equations.
they are mashing up my mind

1) sin2x + sinx * cosx = 1

2)1+cosx + cos(x/2) =0

3) 1 + cos(π +x ) + cos(π/2 + x/2) =0
• Dec 5th 2012, 08:42 AM
HallsofIvy
Re: trig equations
So you just want someone else to do them for you?
• Dec 5th 2012, 09:12 AM
enea54
Re: trig equations
Quote:

Originally Posted by HallsofIvy
So you just want someone else to do them for you?

I am doing a whole summary of the trigonometry chapter at the moment , so that i'll be more prepared for tha exam. These 3 equations were the only ones i couldn't solve , i asked help from my parents , my brother but they couldn't solve it either. I wasn't left with much choices so i went online and posted them here and i think this is the most adequate place to do it. No, i'm not asking someone else to do them for me . I just want a hint, a way of doing them so that i can learn something and put it to use later.
• Dec 5th 2012, 09:29 AM
topsquark
Re: trig equations
Quote:

Originally Posted by enea54
1) sin2x + sinx * cosx = 1

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

$(sin^2(x) -1) + sin(x)~cos(x) = 0$

$-cos^2(x) + sin(x)~cos(x) = 0$

I'd multiply both sides by -1, but you can do it from this too. Factor out the common cos(x) and the solution follows easily from there.

-Dan
• Dec 5th 2012, 09:35 AM
enea54
Re: trig equations
thanks a lot , that was what i needed, thanks
• Dec 5th 2012, 09:47 AM
ebaines
Re: trig equations
For the second one you can replace 'x' with a new variable 'w' where x = 2w. This gives

$1 + \cos(2w) + \cos(w) = 0$

Now use the identity $cos(2w)= 2cos^2w-1$ and solve for cos(w), and from that determine values for w, then x.
• Dec 5th 2012, 09:49 AM
topsquark
Re: trig equations
Ya beat me ebaines! Almost word for word. (Nod)

-Dan
• Dec 5th 2012, 09:54 AM
Plato
Re: trig equations
Notice that $sin^2(x) + sin(x)~cos(x) = 1$

can be written as $\cos(x)[\sin(x)-\cos(x)]=0$
• Dec 5th 2012, 10:26 AM
Soroban
Re: trig equations
Hello, enea54!

Here's the last one . . .

Quote:

$(3)\;1 + \cos(x + \pi) + \cos\left(\tfrac{x}{2}+\tfrac{\pi}{2}\right) \:=\:0$

We have: . $1 + \cos(x + \pi) + \cos\left(\tfrac{x+\pi}{2}\right) \:=\:0$

Let $\theta \:=\:x + \pi$

We have: . $1 + \cos\theta + \cos\tfrac{\theta}{2} \:=\:0$

n . $1 + \left(2\cos^2\!\tfrac{\theta}{2} - 1\right) + \cos\tfrac{\theta}{2} \:=\:0$

. . . . . . . . . $2\cos^2\!\tfrac{\theta}{2} + \cos\tfrac{\theta}{2} \:=\:0$

n . . . . . $\cos\tfrac{\theta}{2}\left(2\cos\tfrac{\theta}{2} + 1\right) \:=\:0$

$\cos\tfrac{\theta}{2} \:=\:0 \quad\Rightarrow\quad \tfrac{\theta}{2} \:=\:\begin{Bmatrix}\frac{\pi}{2} \\ \\[-4mm] \frac{3\pi}{2} \end{Bmatrix} \quad\Rightarrow\quad \theta \:=\:\begin{Bmatrix} \pi \\ 3\pi \end{Bmatrix}$

. . . . $x + \pi \:=\:\begin{Bmatrix}\pi \\ 3\pi\end{Bmatrix} \quad\Rightarrow\quad \boxed{x \:=\:\begin{Bmatrix}0 \\ 2\pi\end{Bmatrix}}$

$2\cos\tfrac{\theta}{2} + 1 \:=\:0 \quad\Rightarrow\quad \cos\tfrac{\theta}{2} \:=\:-\tfrac{1}{2} \quad\Rightarrow\quad \tfrac{\theta}{2} \:=\:\begin{Bmatrix}\frac{2\pi}{3} \\ \\[-4mm] \frac{4\pi}{3} \end{Bmatrix} \quad\Rightarrow\quad \theta \:=\:\begin{Bmatrix}\frac{4\pi}{3} \\ \\[-4mm] \frac{8\pi}{3} \end{Bmatrix}$

. . . . $x + \pi \:=\:\begin{Bmatrix}\frac{4\pi}{3} \\ \\[-4mm] \frac{8\pi}{3} \end{Bmatrix} \quad\Rightarrow\quad \boxed{x \:=\:\begin{Bmatrix} \frac{\pi}{3} \\ \\[-4mm] \frac{5\pi}{3} \end{Bmatrix}}$

• Dec 5th 2012, 11:39 AM
enea54
Re: trig equations
(Bow)(Bow)thanks a lot guys, you are the best of the best , thanks again(Bow)(Bow)(Bow)