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I have forgotten the formulas of circles. Can anyone explain the answer to me?

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Archit3110
radius = 4 = RS= ST
angle = 120 *2 ; 240
2 * 4* pi * 240/360 = \(\frac{16 \pi}{3}\)
IMO C


Bunuel

The center of the circle is 0, and RS = ST = 4. What is the length of arc RWT?


A. \(\frac{4 \pi}{3}\)

B. \(\frac{8 \pi}{3}\)

C. \(\frac{16 \pi}{3}\)

D. \(4 \pi\)

E. \(8 \pi\)


Attachment:
1.jpg

How do you know that R is the radius in this case?
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Bunuel

The center of the circle is 0, and RS = ST = 4. What is the length of arc RWT?


A. \(\frac{4 \pi}{3}\)

B. \(\frac{8 \pi}{3}\)

C. \(\frac{16 \pi}{3}\)

D. \(4 \pi\)

E. \(8 \pi\)


Attachment:
1.jpg

Can anybody explain this one , how is the radius = 4 ?
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Archit3110
radius = 4 = RS= ST
angle = 120 *2 ; 240
2 * 4* pi * 240/360 = \(\frac{16 \pi}{3}\)
IMO C


Bunuel

The center of the circle is 0, and RS = ST = 4. What is the length of arc RWT?


A. \(\frac{4 \pi}{3}\)

B. \(\frac{8 \pi}{3}\)

C. \(\frac{16 \pi}{3}\)

D. \(4 \pi\)

E. \(8 \pi\)


Attachment:
1.jpg


How have you considered radius to be 4?

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From the image below Quadrilateral RWTS is cyclic quadrilateral (CQ) and in a CQ sum of opposite angles is 180
∴ ∠RWT + ∠RST = 180
∠RWT = 180-120 = 60

Another property, if angle subtended by arc RST at the center is θ then the angle subtended in the remaining arc will be θ/2. The opposite is also true.
Since, angle made by arc RST in the remaining arc is 60 degree then the angle subtended at the center by the same arc is 2*60 = 120

And △s ROT and RST are isosceles triangles and if we drop a perpendicular from the point of intersection of two equal sides then the perpendicular:
1. Bisects the angle
2. Bisects the base
3. Perpendicular to the base

Therefore, we can see that △s ROS and OST are equilateral △s as all the angles are 60 degree and since, RS = 4 therefore radius = OR = OS = 4

Length of arc RWT = 2πrθ/360

θ = 360-120 = 240
r = 4

Length of arc RWT = 2π*4*240/360 = 16π/3 (C)
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