1. **Stating the problem:** We need to find the area of the shaded circular segment (tembereng) in circle O with radius $21$ cm. 2. **Understanding the figure:** The radii $OA$ and $OB$ are perpendicular, forming a right angle of $90^\circ$ at the center $O$. 3. **Formula for the area of a circular segment:** The area of the segment is the area of the sector minus the area of the triangle formed by the two radii and the chord. 4. **Calculate the area of the sector:** The sector angle is $90^\circ$, so the sector area is $$\text{Sector area} = \frac{\theta}{360^\circ} \times \pi r^2 = \frac{90}{360} \times \pi \times 21^2 = \frac{1}{4} \times \pi \times 441 = 110.25\pi$$ 5. **Calculate the area of the triangle $OAB$:** Since $OA$ and $OB$ are perpendicular radii, triangle $OAB$ is a right triangle with legs $21$ cm each. $$\text{Triangle area} = \frac{1}{2} \times 21 \times 21 = \frac{1}{2} \times 441 = 220.5$$ 6. **Calculate the area of the segment:** $$\text{Segment area} = \text{Sector area} - \text{Triangle area} = 110.25\pi - 220.5$$ 7. **Evaluate the numerical value:** Using $\pi \approx 3.14$, $$110.25 \times 3.14 = 346.185$$ So, $$\text{Segment area} = 346.185 - 220.5 = 125.685 \approx 126$$ **Final answer:** The area of the shaded segment is approximately **126 cm²**.