Q.The area of a circle is $81\pi$ sq.cm. Find the length of the arc subtending an angle of $300°$ at the centre and also the area of the corresponding sector.

Step 1: $\pi r^2=81\pi \Rightarrow r=9$ cm.

Step 2: $\theta=300°\times\frac{\pi}{180}=\frac{5\pi}{3}$ radian.

Step 3: Arc length $=r\theta = 9\times\frac{5\pi}{3}=15\pi$ cm. …

Find r from the circle's area, convert the central angle to radian, t …

• Simplification slip turning 300×π/180 into 5π/3.
• Using the wrong r (confusing diameter and radius) after solving π …