An arc of a circle of radius 2 cm makes an angle of 90° at the centre. How many cm long is the arc?

Think it through first, then pick an option.

Answer and explanation

Correct answer: a π cm

s = rθ = 2 × π/2 = π cm (90° = π/2 radians).

Why the other options are wrong

b 3π/2 cm: 3π/2 would need θ = 3π/4 (135°); here the angle is 90°.
c 2π cm: 2π would need θ = π (180°); it is a different quantity from the one asked for.
d 4π cm: 4π = 2πr, the whole circumference (360°).