# Trigonometry: finding an angle, area and length of sector of a circle

Homework Statement:
Hello I have come across the question below while revising some trigonometry problems, however, I think I have misunderstood the question because I feel that I have too easily arrived at solutions. I must have missed something or be omitting a method to my workings, I would be very grateful of any guidance A circle has centre O and a radius of 8cm. The arc AB on the circumference is 11cm long.

1. What is the size of angle AOB in radians?
2. What is the area of the sector AOB?
3. Find the length of the line AB.
Relevant Equations:
s= θr
A=1/2r^2θ
1. Using the formula for the arc length; s= θr
I have endeavoured to find the angle AOB sine both the arc length and radius are known;
11= θ*8
θ=11/8=1.375 rad

I actually do not think that this can be correct as it seem to simplistic a response. Have I misinterpreted the question or used the wrong formula? Would it be that the arc length is not 11cm?
Is there a more appropriate method I could adopt to concisely answer this question?

2. Area of a sector = 1/2r^2 θ
This will only be correct if θ is indeed 1.375 rad then,
A=1/2*8^2*1.375=44 cm^2

Again, is there a better approach I could take here?

3. So to find the length of the line AB is would this be the base forming a triangle AOB. If we divide this into two forming triangle AOD, where the midpoint between A and B is D then one can use trigonometry to find the side AD.
The known angle is 1.375 rad / 2 = 0.6875 rad.
sin 0.6875 = AD/8
AD=8*sin 0.6875
AD=5.07685... cm
Since AB = 2AD
AB=2*5.07685~10.2 cm to 3.s.f

I feel that I have oversimplified all of the above problems and arrived at too easy a solution, surely I must be doing something wrong? OI would be very grateful for any guidance.

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## Answers and Replies

BvU
Science Advisor
Homework Helper
surely I must be doing something wrong?
Why this lack of confidence ? It's an introductory exercise. Check your work and be confident if it appears OK !

Why this lack of confidence ? It's an introductory exercise. Check your work and be confident if it appears OK !
Thank you for your reply. Well to be perfectly candid the text book I found it in also accompanied how many marks each section was worth. For example for question 3 that is supposedly worth 5 marks but I cannot see the level of my workings warranting five points. I thought perhaps that I had missed a crucial detail or overlooked a necessary component of the question. Perhaps it is that my method is not applicable here and that there is a more fitting alternative?