# Calculating the angle of rotation

by Maxxx   Last Updated September 11, 2019 10:20 AM

So i have the figure below:

and i want to find the angle $$a$$ as shown in green. I have the coordinates of the blue points where

$$x_i,y_i = (4,2)$$ and $$x_j,y_j = (4.3589, 1)$$

According to the answer, $$a$$ is 13.6441 degrees, but i do not know how to work it out.

I used the formula

but I'm not getting the same answer.

Tags :

Your formula is fine, but you must switch your points, because $$(x_1,y_1)$$ in that formula refers to the point before rotation, which in your case is $$(4.3589, 1)$$, while $$(x_2,y_2)$$ refers to the point after rotation, which in your case is $$(4, 2)$$.