Question: what angle does an arc 5.5cm in length sudtend at the centre of a circle of diameter 7cm
The formula to use is: s=rθ
where the subtended angle, θ, is given in radians.
s = 5.5. cm
r = 3.5 cm
Then θ = s/r = 5.5/3.5 = 11/7, i.e. θ ≈ π/2
Answer: θ = π/2 rads.