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The area of C is the area A of the region obtained by rotating r about the origin from 0 to 2π radians, or from 0° to 360° (like the second hand of a clock in reverse). 

Let θ = the angle between any radius r and the x-axis. If we divide A into many equal sectors, each with arc-length Δθ , each sector has area ΔA = r² Δθ. As we keep dividing A into more and more sections of smaller arc-length, then, as Δθ → 0, the "area" of the sector ΔA → r. 

So, we integrate r dθ from 0 to 2π radians: 

A = ∫ r dθ 

= r² θ / 2 | θ = 0 to 2π 

= (2π r² / 2) - 0 

= π r²
1 5 1
can u please explain this in a simpler method
Radius square =area of circle
diameter divided by 2 = area of circle
1 5 1
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