I Confusion in coordinates

1. Jun 14, 2017

Shafia Zahin

In the attached pic,it is shown that the coordinates of point B are (a cos theta, a sin theta) ,but shouldn't it be (-a cos theta,a sin theta)? Can anybody please help?

Attached Files:

• IMG_20170614_142305.jpg
File size:
72.2 KB
Views:
30
2. Jun 14, 2017

Fightfish

No there is nothing wrong with the labeling of the coordinates in the diagram. Recall that $\cos \theta$ is negative for $\pi/2 < \theta < 3\pi/2$ (or what you may know as the second and third quadrants).

3. Jun 14, 2017

Shafia Zahin

But didn't it come like this?(see the attachment)

Attached Files:

• IMG_20170614_144504.jpg
File size:
46.7 KB
Views:
23
4. Jun 14, 2017

Fightfish

When you did the triangle construction in your diagram, you treated $x$ there as a length, which only takes on positive values, but ignored its position relative to where the origin was defined. So, the x-coordinate of the point should in fact be the negative of the $x$ in your derivation.

5. Jun 14, 2017

Shafia Zahin

Oh,now I got it,thank you so much