# Calculating the radius of a circle - simple trigonometry/geometry

• Apr 9th 2013, 01:08 PM
Mukilab
Calculating the radius of a circle - simple trigonometry/geometry
From the circle detailed in this video:
Edexcel Core Maths C2 June 2011 Q5b : ExamSolutions - YouTube

To find the radius of the inner circle, why cannot we take the line down from OC to form a right angled triangle, and then do
tan(pi/6)=r/6?

Since tan is opposite over adjacent, why would this not work?

Apparently the answer is r=2, that's now what you would get doing the above. Why is it wrong?
• Apr 9th 2013, 06:20 PM
chiro
Re: Calculating the radius of a circle - simple trigonometry/geometry
Hey Mukilab.

I don't know what triangle you are referring to, but it isn't in any way reflecting the radius of the circle. What the video did is the best way to figure out the problem IMO.
• Apr 9th 2013, 10:19 PM
Mukilab
Re: Calculating the radius of a circle - simple trigonometry/geometry
Ah nevermind, I started drawing the circle/triangle on paint and I realised my mistake.

Thank you for taking the time to post anyway :)