The area of any circle is (pi) x (radius)² .
The area of the WHOLE circle in the question is
(pi) x (2.5 cm)²
= (pi) x (6.25 cm²) .
The full circle is made by 360° of central angles.
90° of central angle makes a pie-wedge (sector)
that's exactly one fourth of the circle, so its area
is one fourth of the area of the whole circle.
Area = (1/4) (pi) (6.25 cm²)
= (0.25) (3.14) (6.25 cm²)
= 4.906 cm² .
Rounded to the nearest tenth: 4.9 cm² .
