# Getting angle using tangent

here's the code (https://code.sololearn.com/WY6NL88keDO6/?ref=app) . The red circle is at a known angle of 130°, then I want to draw the navy line from the center to 130° using x and y of the red circle but it looks like I missed the calculation. Currently, the angle of the Navy line is a reflection to the angle of the red line and if I add minus sign ➖ to **diffX* * at line13, it'll work as expected but Why do I need to do that by myself, why can't the Calculations at line 10 and 13 figured out if x shouod be minus ➖ or plus I couldn't figure out where I was wrong

6/2/2020 3:43:36 PM

Mirielle [INACTIVE]5 Answers

New Answerlet diffX = (W/2) - x; let diffY = (H/2) - y; let dist = Math.hypot(diffX, diffY); // pythagoras let unknownAngle = Math.atan2(y - H/2, x - W/2); let newX = (W/2) + Math.cos(unknownAngle) * dist; let newY = (H/2) + Math.sin(unknownAngle) * dist; something like this probably? (im bad at math..so,if this wrog,just ignore it😅)

this works for me. let diffX = x - (W/2); let diffY = (H/2) - y; let dist = Math.hypot(diffX, diffY); // pythagoras let unknownAngle = -Math.atan2(diffY, diffX); in cartesian coordinates (-, +) | (+, +) ———————— (-, -) | (+, -) Explanation: (subtraction with W/2 thing is abit tricky here) your diffX is giving positive value where it should be negative and vice versa and as the result we are getting symmetric in them.

Lily Mea your solution would behave like the angle is a reflection to the angle. However, if I use your solution and then add "-" to diffX at line13, it'll work.. Now, ik why that works but I'm still confused why do I need to add "-" by myself and the calculations on line10 won't

but why do I need to add it manually, I'm expecting line10 to output a plus or minus result edit: RKK , now I know why it works, calculating the difference should be (final - initial) not (initial - final), if the angle is below the center of the graph then second angle *y* should be y - (H/2)