# Thread: How to eliminate angle from equations.

1. ## How to eliminate angle from equations.

Hi there, I have to eliminate angle φ from equations:

And transform to the equation of circle:

2. ## Re: How to eliminate angle from equations.

Hello, blueye89!

There is a typo . . . one of the equations must have a minus-sign.

I have to eliminate $\displaystyle \theta$ from these equations:

. . $\displaystyle x \:=\:\tfrac{15}{2} + \tfrac{3}{2}\cos2\theta + 2\sin2\theta$

. . $\displaystyle y \:=\: \tfrac{3}{2}\sin2\theta\,{\color{red}-}\,2\cos2\theta$
n . . . . . . . . . . . $\displaystyle {\color{blue}\uparrow}$

And transform to the equation of circle: .$\displaystyle \left(x-\tfrac{15}{2}\right)^2 + y^2 \:=\:\left(\tfrac{5}{2}\right)^2$

We have: .$\displaystyle \begin{Bmatrix}x-\frac{15}{2} &=& 2\sin2\theta + \frac{3}{2}\cos2\theta \\ \\[-4mm] y &=& \frac{3}{2}\sin2\theta - 2\cos2\theta \end{Bmatrix}$

Square both equations: .$\displaystyle \begin{Bmatrix}(x-\frac{15}{2})^2 &=& 4\sin^2\!2\theta + 6\sin2\tyheta\cos2\theta + \frac{9}{4}\cos^2\!2\theta \\ \\[-3mm] y^2 &=& \frac{9}{4}\sin^2\!2\theta - 6\sin2\theta\cos\theta + 4\cos^2\!2\theta \end{Bmatrix}$

Add: .$\displaystyle \left(x-\tfrac{15}{2}\right)^2 + y^2 \;=\;\tfrac{25}{4}\cos^2\!2\theta + \tfrac{25}{4}\sin^2\!2\theta$

n . . . $\displaystyle \left(x-\tfrac{15}{2}\right)^2 + y^2 \;=\;\tfrac{25}{4}\underbrace{(\sin^2\!2\theta + \cos^2\!2\theta)}_{\text{This is 1}} \;=\;\tfrac{25}{4}$
n . . . $\displaystyle \left(x-\tfrac{15}{2}\right)^2 + y^2 \;=\;\left(\tfrac{5}{2}\right)^2$

3. ## Re: How to eliminate angle from equations.

Yes my friend. Sorry,my mistake, there is an minus in second equation. Thank you very much.