The measure of central angle abc is π/2 radians. what is the area of the shaded sector

For this case we have that the area of the shaded region, is given by:
 
 Where,
 s' / s: relationship between central angle and full angle of the circle
 r: radius of the circumference.
 Substituting values we have:
 
 
 The area of the shaded sector is:
 
 option 2

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